Common questions
Questions from developers, planners, and architects about civil engineering, consents, and working with SAE Ltd.
Stormwater neutrality means that after development, the peak stormwater discharge rate from your site must not exceed the pre-development rate. Most councils in New Zealand now require this for residential subdivision and infill development.
Napier City Council introduced its SW-S1 rule in 2025. Auckland Council has required stormwater neutrality under its Unitary Plan for some years. Rangitikei District Council and Horizons Regional Council have their own requirements. The specific rule — what storm events must be matched, which return periods apply, and what detention systems are acceptable — differs between councils.
SAE has applied stormwater neutrality rules under Napier City Council, Auckland Council (TP108), Rangitikei District Council, Horizons Regional Council, and Hamilton City Council (PC12).
A detention tank is sized to store stormwater during a design storm event and release it slowly enough that the post-development peak flow at the point of discharge matches the pre-development rate. The sizing process uses HEC-HMS hydrological modelling: rainfall data from NIWA HIRDS V4 at the site's location and return period, the SCS runoff method (or TP108 variant for Auckland), and storage routing through the tank with an orifice outlet.
The orifice diameter is calculated to limit the outflow rate to the pre-development level. The required storage volume follows from the difference between inflow and permitted outflow over the duration of the design storm.
Tank volumes in completed SAE projects range from 2,000 litres per lot for small infill to 16,000 litres for a 6-unit development (Barker Rd, Napier) and up to 1,020 m³ for a 290 m detention swale (George St, Bulls).
Each territorial authority sets its own stormwater rules in its district plan or engineering standards, often in conjunction with the regional council's discharge rules. The design storm event, the allowable discharge rate, the acceptable detention system types, and the modelling method — Rational Method, SCS, or Auckland TP108 — all vary.
For example: Napier City Council's SW-S1 rule caps post-development discharge at the pre-development rate for 10-year and 100-year events. Auckland Council's Unitary Plan requires TP108 SCS modelling, which uses a more conservative initial abstraction of Ia = max 5mm versus the standard ? = 0.2. Horizons Regional Council may require a separate resource consent for a discharge to a watercourse, independent of the territorial council's requirements.
SAE holds experience with the specific requirements of each council it operates in, and confirms the applicable rules at the fee proposal stage before any design work begins.
Yes, where ground conditions allow. Soakage systems are an alternative to detention tanks where the soil permeability is high enough and the groundwater table is deep enough to accommodate a pit of the required capacity without flooding or surfacing.
S-Map soil data is used to identify soil type and drainage capability. Field soakage tests (falling head method) are used to confirm the design rate. In the Bay of Plenty, pumice soils can achieve soakage rates of 800–1,720 mm/hr, making soakage the preferred option — as was the case at a commercial farm in Otakiri. In Rangitikei, Pallic soils (Flaxtonford series) are poorly drained, and soakage is generally not viable; kerb discharge with detention is used instead (Flower St, Bulls).
Groundwater depth can also constrain pit depth. At the Otakiri site, groundwater at 1 m limited pit depth to 0.8 m maximum, which affected the required soakage base area and pit configuration.
A flood hazard assessment is typically required when: (a) your site is identified in a flood hazard overlay in the district plan or regional council flood maps, (b) a resource consent condition requires a flood report before development can proceed, (c) a building consent application requires confirmation of design floor levels, or (d) you are purchasing land and want to understand flood risk before committing.
1D (one-dimensional) flood modelling analyses flow along a defined channel or pipe cross-section using Manning's equation. It is appropriate for rivers and drains where flow is channelled in a clear direction, and where flood levels are needed at specific cross-sections.
2D (two-dimensional) modelling solves flow equations across a grid, producing depth and velocity maps across a floodplain or urban surface. It captures overland flow paths that do not follow a defined channel — which is common in urban infill situations where stormwater moves across lots, through fences, and along footpaths rather than down a single waterway.
For Auckland infill and urban sites where overland flow paths are complex, 2D HEC-RAS modelling on a 1 m × 1 m LiDAR grid is used. The Glendale Rd, Auckland study is an example: two overland flow paths crossing the site were modelled at 0.469 m³/s and 4.28 m³/s, with depths to 1.2 m and pedestrian hazard areas mapped.
The Soil Conservation Service (SCS) runoff curve number method is the standard approach for estimating catchment peak flows in New Zealand. Three variants are in common use:
The variant chosen significantly affects the calculated flow. On the Tukituki River assessment, the three variants returned 100-year flows of 894 m³/s (Original SCS), 1,129 m³/s (Brazilian SCS, +21%), and 1,186 m³/s (TP108, +25%). The most conservative was adopted for design. Cyclone Gabrielle 2023 subsequently validated this approach: surrounding properties flooded, but the assessed lots did not.
A finish floor level (FFL) is the minimum elevation at which the ground floor of a building must be constructed, set above the design flood level with an allowance for freeboard. In flood-prone areas, councils specify a design flood event — typically 100-year, or 200-year in some Horizons Regional Council jurisdictions — plus a freeboard margin above the calculated flood surface.
SAE calculates FFLs from flood level assessments using LiDAR terrain and hydraulic modelling. Datum reconciliation is part of this work: at Hereford Heights, Marton, the survey datum was WVD1953 while HBRC flood data uses NZVD2016 — a 0.543 m difference that must be identified and corrected before any FFL recommendation is made.
The FFL recommendation is included in the engineering report and can be provided directly to the building consent applicant or designer.
Resource consent for subdivision typically requires an infrastructure report or engineering assessment covering: stormwater management (neutrality calculation, pipe network, overland flow paths), wastewater (network design or connection to existing), water supply (reticulation and fire-fighting supply verification per SNZ PAS 4509:2008), roading (geometry, pavement design, sight distances), and earthworks (cut/fill volumes, erosion and sediment control plan).
The specific requirements depend on the council, the zone, and the scale of development. For large subdivisions in Rangitikei — such as Hereford Heights (90 lots) and Henderson Line (74 lots) — full NZS 4404:2010 infrastructure design is required. For small Auckland infill (2–5 lots), the Auckland Stormwater Code of Practice v4 2024 and Watercare Code of Practice 2023 govern the scope.
SAE can advise on what will be required for your specific project and council as part of the initial enquiry response.
S224c refers to Section 224(c) of the Resource Management Act 1991 — the certificate issued by the council confirming that all conditions of the subdivision consent relating to engineering works have been met. Without an S224c certificate, the subdivision titles cannot be issued by Land Information New Zealand (LINZ).
The S224c application requires as-built drawings, producer statements, council inspection records, and confirmation that all engineering works have been constructed in accordance with the consent and approved plans.
SAE has issued S224c packages for Henderson Line, Marton (Stages 1 and 2, 2025), Hereford Heights, Marton (multi-stage, 2019–2022), and Rayner Lane, Marton (January 2026).
Yes. SAE holds active project experience in Auckland (direct projects and CPEng review work), Waikato (Hamilton), Bay of Plenty (Whakatane), and Taranaki (Stratford). Projects outside Hawke's Bay and Manawatu are considered on enquiry.
The primary factors are whether SAE can access the necessary LiDAR and council data remotely, and whether the project type matches SAE's services. Most flood assessments and stormwater neutrality reports can be completed without a site visit if adequate survey data and LiDAR are available.
For projects requiring construction supervision or as-built survey, travel and accommodation are included in the fee proposal.
NPS-FW is the National Policy Statement for Freshwater Management 2020, a national direction document that requires councils to protect natural wetlands and waterways. Under NPS-FW and the associated National Environmental Standards for Freshwater (NES-FW), natural wetlands must be avoided, or development requires consent with a very high threshold for approval.
If your site contains or is adjacent to a natural wetland, development may be restricted. However, not every wet area on a site qualifies as a natural wetland under the NES-FW definition.
At Kensington Road, Marton, a specialist firm classified a pond on site as a natural wetland. SAE rebutted this classification using aerial photographs from 1942, 1949, and 1982, demonstrating that the pond appeared only after 1982 and was therefore not a pre-existing natural wetland. The rebuttal was accepted, allowing the subdivision to proceed. If a wetland classification is raised on your site, early investigation of the historical evidence is important.
CPEng stands for Chartered Professional Engineer — a credential awarded by Engineering New Zealand to engineers who demonstrate competency, experience, and ongoing professional development to a defined national standard. CPEng is assessed by a panel of peer engineers and requires a minimum of seven years of relevant experience. The credential is listed on the Engineering New Zealand public register and is recognised across New Zealand, Australia, and internationally through mutual recognition agreements.
For Auckland Council resource consent applications involving civil infrastructure, a CPEng sign-off is required for the engineering report. For stopbank design, council procurement, and infrastructure projects above a certain scale, CPEng is typically required or strongly preferred by the consenting authority or project client.
Andre Magdich attained CPEng in 2023 and is a Chartered Member of Engineering New Zealand.
A peer reviewer (or reviewing engineer) independently reviews an engineering report or design prepared by another engineer, checks it for technical correctness and compliance with applicable standards, and issues either a concurrence letter or a list of required amendments. The reviewer takes professional responsibility for the sign-off.
Where a CPEng sign-off is required — for example, many Auckland Council resource consent applications involving civil infrastructure — the reviewing engineer must hold CPEng status.
SAE acts as reviewing and authorising CPEng engineer for Auckland-based consultancies, reviewing approximately 30 infill residential infrastructure reports per year. SAE has also reviewed reports prepared by Resonant Consulting (Bulls Development, 93 lots) and LANDEV Consulting.
Yes. SAE Ltd holds professional indemnity insurance through NZI. Details can be provided on request for project procurement or pre-qualification purposes. Contact info@subdivisionengineering.nz to request a certificate of currency.
Email info@subdivisionengineering.nz or call 027 604 4015. Include the site address, the type of development, your council, and your timeframe. If you have existing consent conditions or engineering reports, attach those.
An initial response is provided within one business day confirming whether SAE can assist and what information is needed to issue a fee proposal.
SAE issues fixed-fee or capped-rate proposals. The fee is based on the scope of work: report type, number of lots or units, modelling complexity, number of councils involved, and expected RFI rounds.
Most residential stormwater neutrality reports (2–6 lots) are fixed fee. Flood assessments and subdivision infrastructure reports may be capped, with a schedule of hourly rates for work beyond the agreed scope. Fees are confirmed in writing before any work begins.
SAE does not charge for initial enquiries.
Typical turnaround from receiving all required data:
These timeframes are from data receipt, not from the date of enquiry. Urgent projects are accommodated where possible — discuss your timeframe at enquiry stage.
Not always. Many stormwater neutrality reports and flood assessments can be completed using LINZ LiDAR, S-Map soil data, NIWA HIRDS rainfall data, and council-supplied network information, without a site visit.
A site visit is undertaken where: ground conditions or overland flow path behaviour cannot be determined from LiDAR alone, there are existing structures affecting the drainage design, construction supervision is required by consent conditions, or the council requires a site inspection before issuing engineering plan approval.
Where a site visit is required, it is identified at the fee proposal stage and included in the quoted scope.
Send an enquiry and SAE will respond within one business day.
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