Water New Zealand – Developing Aotearoa’s First National Stormwater Modelling Guide

Summary

• In 2020, Water New Zealand partnered with Awa Environmental and other technical specialists to develop the country’s first National Stormwater Modelling Guide, a landmark initiative designed tobring consistency, clarity, and best‑practice methodology to urban stormwater modelling across Aotearoa.
• The sector had long faced fragmented guidance, inconsistent modelling approaches, and gaps in technical advice.
• Water New Zealand coordinated a multi‑year, nationwide effort to address these challenges, working alongside councils, consultants, government agencies, and industry specialists.
• Awa served as a technical lead, co-author, and co-editor, helping shape the guide’s structure, modelling principles, and recommended processes.
• The result is a comprehensive national reference that supports better flood‑risk understanding, more reliable modelling outcomes, and improved decision‑making for growing communities and thriving ecosystems.
• The final publication – launched in 2024 and now adopted across the industry – provides a robust foundation for future modelling work, improves national capability, and strengthens resilience planning in a changing climate.

Challenges

• Before this project, New Zealand had no unified national standard for urban stormwater modelling.
• Councils, consultants, iwi, land developers, and Crown and Māori agencies relied on a patchwork of regional guidelines, legacy practices, and varying levels of technical detail.
• This inconsistency created challenges for procurement, model review, flood‑hazard mapping, and long‑term infrastructure planning.
• The sector also faced increasing pressure from climate change, urban intensification, and heightened community expectations for transparent, defensible flood‑risk information.
• The Modelling Special Interest Group at Water New Zealand first recognised the need for a national guide that could be trusted by practitioners, regulators, and the public.
• The project required navigating diverse regional practices, integrating multiple modelling methodologies, and ensuring the final guidance was technically rigorous yet accessible to a broad audience.

Solutions

• Collaborating with others, Awa Environmental led the technical development of the guide from inception to publication, leveraging decades of industry experience and deep expertise.
• The work included:
  
  • Defining the national modelling framework: Plan, Build, Use, Share, and Maintain.
  • Establishing core modelling principles, ensuring the guidance was future‑proof, non‑prescriptive, and adaptable to innovation.
  • Developing detailed methodologies for hydrology, hydraulics, boundary conditions, quality assurance, and model confidence assessment.
  • Facilitating extensive engagement, including industry surveys, workshops, interviews, and collaboration with regional councils, iwi, land developers, consultants, and government agencies.
  • Integrating best‑practices from across New Zealand and international sources.
• The team combined scientific rigour, practical experience, and deep understanding of regulatory and operational realities, ensuring the guide was technically robust while remaining usable for practitioners and other users.

Benefits

• The National Stormwater Modelling Guide is now a cornerstone reference for the sector.
• It delivers:
  
  • National consistency, reducing ambiguity and improving the quality of modelling outputs.
  • Better flood‑risk understanding, supporting the protection of assets and infrastructure and the building of more resilient communities and thriving ecosystems.
  • Improved procurement and review processes, saving time and reducing project risk.
  • A shared language and methodology, strengthening collaboration across councils, consultants, land developers, iwi, and Māori and Crown agencies.