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Understanding Plan Change 120

Plan Change 120 (PC120) to the Auckland Unitary Plan is a significant update notified by Auckland Council in November 2025. Titled "Housing Intensification and Resilience", it enables more housing growth in appropriate locations while introducing a stronger risk-based framework for managing natural hazards, particularly flooding.

Key Changes for Engineers

PC120 does not change the existing stormwater management requirements. SMAF (Stormwater Management Area — Flow) provisions, retention and detention volume calculations, and water quality treatment rules continue to apply exactly as before under the current Auckland Unitary Plan.

However, PC120 introduces important new natural hazard provisions (Chapter E36) that are highly relevant to stormwater design:

• Updated flood hazard mapping and risk categories
• Stronger protection for floodplains and overland flow paths
• Greater emphasis on ensuring new development does not increase flood risk to other properties or the environment
• Enhanced requirements for site-specific flood risk assessments

Impact on Current Projects

The new natural hazard provisions have legal effect from the date of notification. Projects already in the resource consent pipeline should be reviewed against the updated flood hazard mapping and rules. Existing stormwater requirements (including SMAF) continue to apply in full alongside these new provisions.

What Should You Do Now?

We recommend reviewing any projects in early design stages against the official PC120 natural hazard mapping and provisions. This is particularly important for sites in or near flood-prone areas, streams, or existing SMAF zones.

Our SMAF Calculator on the Flowpath Portal is there to help with SMAF calculations and make stormwater assessments easier.

For the latest official details and maps, refer directly to Auckland Council's PC120 resources.

A Practical Guide for Civil Engineers

The National Environmental Standards for Freshwater (NES-FW), which came into effect in September 2020 with several subsequent amendments (including major clarifications in 2022 and further updates effective January 2026), set nationwide rules for activities that pose significant risk to freshwater and freshwater ecosystems. For civil engineers involved in subdivision, land development, and infrastructure projects, a clear understanding of these standards is essential.

What Does NES-FW Cover?

The standards focus on protecting freshwater values through rules for specific high-risk activities:

• Natural inland wetlands — Vegetation clearance, earthworks, structures, and drainage works in or near natural inland wetlands generally require resource consent and often have a higher activity status than under previous rules.
• Rivers and streams — New or modified culverts, weirs, and stream crossings must provide for fish passage (including many intermittent and ephemeral streams).
• Riverbed works — Rules to prevent infilling and modification of streams to protect aquatic habitat.

The Wetland Definition Challenge

One of the most contentious aspects of the NES-FW has been the definition and mapping of "natural inland wetlands." The 2022 amendments clarified and narrowed the definition to exclude certain constructed wetlands and pasture areas, but on-site identification can still be challenging. Early ecological assessment is strongly recommended for any site containing low-lying or poorly drained areas.

Practical Design Implications

For stormwater and land development projects, the NES-FW emphasises the effects management hierarchy (avoid, minimise, remediate, offset). This typically means:

• Heightened scrutiny and protection for activities near natural inland wetlands
• Mandatory fish passage design for any structure in or across a river or stream
• Consideration of offsetting or ecological compensation where complete avoidance is not practicable

Using the Portal Tools

Our detention routing calculator and TP108 tools on the Flowpath Portal can help you model and demonstrate that post-development flows and volumes are appropriately managed. This supports compliance with both NES-FW requirements near sensitive freshwater bodies and the relevant regional plan provisions.

Beyond the Rational Method

The Rational Method (Q = C i A) has been a staple of civil engineering for over a century. Its simplicity makes it an excellent tool for quick screening and very small sites. However, for modern stormwater design in Auckland — particularly under TP108, GD01, and the Auckland Unitary Plan — it is generally only the starting point.

Why TP108 and GD01 Matter

Auckland Council's Technical Publication 108 (TP108 – Guidelines for Stormwater Runoff Modelling) together with Guideline Document 001 (GD01 – Stormwater Management Devices in the Auckland Region) provide the accepted methodology that goes well beyond simple peak-flow estimation:

• Hydrograph-based routing — Full inflow hydrographs are generated and routed through detention and retention systems to demonstrate proper attenuation
• Critical duration analysis — Multiple storm durations are tested to identify the worst-case scenario for both peak flows and volumes
• Climate change allowances — Appropriate rainfall intensity increases are applied in line with current projections
• Volume-based requirements — SMAF provisions require both retention (permanent volume removal) and extended detention (slow release), which cannot be accurately sized from peak flow alone

The Role of Continuous Simulation

For larger, more complex, or catchment-scale sites, continuous simulation modelling (using tools such as HEC-HMS or MIKE) provides the most robust assessment. These models evaluate system performance across long-term rainfall records rather than a single design storm.

Practical Tool Selection

For most residential and small-to-medium commercial projects in Auckland, our TP108 calculator and detention routing tools on the Flowpath Portal provide a practical and proportionate level of analysis. They implement the TP108 and GD01 methodologies while making rapid design iteration and resource consent documentation fast and straightforward.

For broader subcatchment or network assessments, more detailed council-accepted models may be required. The Portal tools remain ideal for site-level design and supporting consent applications.

Key Takeaway

The Rational Method still has its place for quick estimates and very small impervious areas. However, any project involving detention devices, SMAF requirements, or resource consent should use hydrograph-based and volume-based methods in line with TP108 and GD01. The Flowpath Portal makes compliant modern design accessible to every engineer.

Kainga Ora Development: A Case Study

Flowpath Engineering was engaged to provide civil engineering and stormwater design services for a significant Kainga Ora (formerly Housing New Zealand) medium-density housing development in Auckland. The project presented a compelling set of challenges: high-density housing targets, a constrained site, and stringent stormwater management requirements under SMAF provisions.

The Challenge

The site — a former single-dwelling residential block — was to be redeveloped with multiple medium-density dwellings under Kainga Ora's urban renewal programme. Key challenges included:

• Significant impervious area increase — From approximately 35% to over 70% site coverage
• SMAF Zone 1 — Requiring both retention (first 5mm) and extended detention (next 29.2mm for the 95th percentile storm) volumes
• Limited site area — Stormwater devices needed to fit within narrow setback areas and shared accessways
• Downstream flooding — Existing downstream network had known capacity issues, making flow attenuation critical

Our Approach

Flowpath used an integrated stormwater approach combining multiple treatment and attenuation methods:

• Rainwater harvesting tanks providing the retention volume (connected to toilet flushing and irrigation to ensure drawdown)
• Below-ground detention tanks with orifice-controlled outlets for extended detention and peak flow attenuation
• Permeable paving in shared accessways contributing to water quality treatment and partial retention

All sizing was verified using the Flowpath Portal's SMAF calculator and detention routing tools, with TP108 methodology applied for the hydrological analysis.

The Result

The design achieved full compliance with Auckland Council's stormwater management requirements while maintaining the development yield. Resource consent was granted without stormwater-related conditions requiring redesign — a direct result of thorough upfront analysis and clear documentation.

Lessons Learned

This project demonstrated that even heavily constrained urban sites can achieve stormwater compliance with good design. Early engagement with stormwater requirements (rather than treating them as an afterthought) was key to avoiding costly redesign later in the process.