Ōmāhu stopbank upgrade

During Cyclone Gabrielle, the Ōmāhu stopbank was breached, resulting in severe flooding across the area. While repairs restored the Ōmāhu stopbank height to the previous regional standard of  1% Annual Exceedance Probability (1% AEP)*, new post-cyclone modelling shows it now falls short.

The design draws on updated post-Gabrielle modelling and includes an additional 700mm of freeboard – a safety buffer to allow for uncertainty and climate extremes.

The jointly funded NIWE programme has given us the opportunity to upgrade the stopbank for this area which will help better protect local homes, businesses and infrastructure – and give people more time to act when the next major storm hits.

1% AEP = there is a 1% chance of an event that size happening in any given year. It’s a probability not a predication, so they can, and do, occur more than once in 100 years.

• Upgrading a 3.2 km section of stopbank along the true left of the Ngaruroro River (from the extended stopbank at Taihape Road to Korokipo Road)
• Reshaping the riverside slope – or batter – to meet current best-practice design standards
• Reviewing vehicle access ramps to reduce erosion risks and overtopping during floods

A scheme is the network of stopbanks, pump stations, floodgates, plantings and other works that reduce flood risk in a particular area. Each scheme is built to an agreed regional standard or ‘level of service’ – the performance standard it is designed to deliver.
In flood control, this is usually expressed as the size of flood it can manage, such as a 1% AEP event. Over time, that standard can be reduced by climate change, earthquakes, or changes in the riverbed. A level-of-service upgrade restores the network to its agreed benchmark – and at times raises it beyond.

Our resource consent was lodged in June 2026 and – all going well – we plan to start construction in early August. We expect the main construction phase to be completed by Christmas, though there’ll likely be some tidying up work after that – reinstatement work, planting, grassing and landscaping.

Meeanee-based company Galbraith Earthmoving is our main construction partner for the project. We’re currently working with them on the Pōrangahau Flood Resilience project: they’re great local people doing mighty local mahi.

Funded by Central Government and HBRC under the North Island Weather Event (NIWE) flood mitigation programme, this project is one of HBRC’s Level of Service upgrades, a programme that has a total budget of $30 million. Most of that cost – 75.15% – is met by Central Government, with the remaining 24.85% funded by HBRC through rates.