The Milne Street Extension site at Kensington Road, Marton (43.57Ha) was the subject of a preliminary infrastructure assessment to support a proposed rural-to-residential rezoning. The assessment needed to demonstrate that the site could physically accommodate residential development at up to 400 lots - covering stormwater, wastewater, water supply, roads, and earthworks - and identify any significant constraints that could affect the rezoning application before it was lodged with Rangitikei District Council.
The site shared several characteristics with the adjacent Kensington Road subdivision: Pallic (Flaxtonford) soils with low permeability, convergence of overland flows from the surrounding catchment, proximity to Folly Stream, and the presence of a pond that had appeared in recent aerial imagery. The NPS-FW 2020 risk was therefore real - if the pond was classified as a natural wetland, setback requirements under NES-FW could sterilise a significant portion of the developable area. At the scale of a 400-lot development, this was a binary planning risk, not a minor constraint. Resolving it before lodgement was necessary to avoid the rezoning application being stalled on a question that could be answered at the desk.
The stormwater challenge at this scale was also significant. A 43.57Ha site transitioning from rural pasture to 400 residential lots involves a fundamental change in surface impermeability and runoff behaviour. The assessment needed to quantify the pre-development and post-development peak flows at each drainage sub-catchment boundary, size detention infrastructure to demonstrate hydraulic neutrality, and assess the downstream capacity of the receiving watercourses - including Folly Stream, which was subject to overtop risk at the Kensington Road crossing. Additional constraints on the site boundary included an active rail line and industrial properties to the south, which influenced the road layout options and the extent of the developable area.
The wetland risk was addressed using the same approach that had been applied on the adjacent Kensington Road consent: historical aerial photographs from Retrolens were sourced for 1942, 1949, and 1982. Analysis of those images confirmed that no pond or wetland feature was present in any of those years. The pond visible in recent aerial imagery had appeared only after 1982. Under NPS-FW 2020, a water body that did not exist prior to human modification - and cannot be demonstrated to be natural - does not meet the definition of a natural wetland to which NES-FW setback provisions apply. This finding was documented with the source aerials and included in the preliminary assessment for direct use in the rezoning application, removing the most significant planning constraint at the feasibility stage.
The drainage catchment was divided into five sub-catchments - designated W1, W2, E1, E2, and E3 - to reflect the natural topographic drainage divides across the 43.57Ha site. Total modelled catchment area was 50,724m². HEC-HMS was used with NIWA HIRDS V4 rainfall data to calculate pre- and post-development peak flows at both the 10-year and 100-year return periods, with climate change applied in accordance with Horizons Regional Council requirements. A combined detention swale system was sized to attenuate post-development flows: 500m³ storage volume against a 388m³ minimum calculated requirement, providing a safety margin appropriate to the scale of the site and the low-permeability soils. A 750mm Class 4 concrete culvert with a 35-metre span was designed to convey overland flows through the site at the primary drainage crossing point.
Wastewater assessment required a preliminary network design for up to 400 lots. Average daily wastewater flow (ADWF) was calculated at 300,000 litres per day; peak flow at 6.94 L/s. The assessment noted that GHD had separately reviewed downstream wastewater network capacity and had identified capacity constraints in the existing reticulation. Those constraints would need to be addressed through either network upgrades or a staged development programme, and both options were identified in the preliminary report. Earthworks volumes were estimated at 24,000m³ cut and 3,500m³ fill for preliminary road and lot formation, based on the indicative lot layout and existing ground contours.
Site area: 43.57Ha. Development potential: up to 400 lots (preliminary, subject to rezoning). Drainage sub-catchments: 5 (W1, W2, E1, E2, E3). Total catchment area: 50,724m². Pre-development 10-year peak flow: 457 L/s. Post-development 10-year peak flow: 711 L/s (an increase of 55%). Post-development 100-year peak flow: 1,203 L/s. Detention swale: 500m³ storage volume provided against a 388m³ minimum requirement, with rated capacity of 1,885 L/s. Primary culvert: 750mm Class 4 concrete, 35-metre span. Wastewater ADWF: 300,000 L/day (400 lots at standard generation rate). Wastewater peak flow: 6.94 L/s. Earthworks estimate: 24,000m³ cut, 3,500m³ fill (preliminary, based on indicative layout). Standards applied: NZS 4404:2010, RDC Code of Practice 2017, NPS-FW 2020, NES-FW, Horizons RC stormwater discharge rules. Software: Rational Method, NIWA HIRDS V4, S-Map, HEC-HMS, Retrolens historical aerials.
Wetland risk: pond present in recent aerials appeared post-1982 only - no natural wetland under NPS-FW 2020. Documented with 1942, 1949, and 1982 Retrolens aerials. Wastewater network: GHD capacity review integrated into assessment; downstream upgrades required before full development can proceed. Soil classification: Pallic (Flaxtonford) soils - no soakage option; stormwater managed via surface detention only. Site boundary constraints: active rail line to south; industrial properties; Folly Stream overtop risk at Kensington Road crossing. Report: April 2023 preliminary infrastructure assessment prepared for the rezoning application.
The preliminary infrastructure assessment confirmed that the 43.57Ha site is physically capable of accommodating residential development at the proposed scale, subject to stormwater detention sized to the calculated requirements, wastewater network capacity upgrades identified by GHD, and compliance with Horizons Regional Council discharge rules. The NPS-FW wetland risk - which had the potential to sterilise a significant portion of the site - was resolved at the desk assessment stage before the rezoning application was lodged, removing the most significant planning constraint from the process at the point in the programme where it costs the least to address.
The assessment also established the infrastructure framework - drainage sub-catchment boundaries, detention swale sizing, wastewater design flows, and earthworks estimates - that subsequent detailed design stages will use as their starting point. The staged wastewater approach, with GHD's network review integrated, gives the rezoning application a credible pathway to service the full 400-lot development potential without relying on network upgrades that are not yet programmed.
Identifying the NPS-FW risk early - and resolving it with historical aerial evidence before the rezoning application was lodged - is exactly what preliminary infrastructure assessments are for. Finding the constraint at feasibility stage costs a fraction of what it costs to discover it during consent.
