Ōtaki wastewater gravity main

We received $29.3 million from the Crown Fund to help improve water and transport systems in Ōtaki. This money will help us make big changes to the whole network, instead of doing one project at a time.

One project involves upgrading a major wastewater pipe in Ōtaki, to make the town’s wastewater system stronger and better for current and future homes. It'll also make the system work better by reducing the number of pump stations needed.

You can read more about this on our A blueprint for Ōtaki page.

Image: We’re laying 2.7 kilometres of new wastewater pipe along Aotaki Street and Mill Road to the Old State Highway 1. See larger image [JPG 489 KB]. 

Progress update

Updated 28 July 2025: This work involves laying a new, bigger wastewater pipe, improving capacity, resilience and future-proofing the wastewater network. We're nearing completion but before we reach the finish line, we have to lay the pipe across two roundabouts on Aotaki Street and across the old state highway.

Work will reach the Aotaki St roundabout in mid- August. The pipe from Aotaki Street will be connected to the pipe already laid on Mill Road. A pipe will also be installed across the roundabout and approximately 50 metres along the northern side of Aotaki Street.

This section of work is expected to take four weeks and will include two main site lay outs, with supporting traffic management. The south arm of the roundabout will be closed for up to two weeks. Then the east arm of the roundabout will be closed to enable the installation of a manhole.

Detours will be in place, and these will change as the work progresses. There will be temporary changes to bus stop locations.

What’s next

In mid-September, the wastewater pipeline on Mill Road will reach the old state highway. This stage of works is expected to take three months and will involve rebuilding the roundabout.

Background

When there's a lot of rain, the Ōtaki wastewater network can flood and overflow. This happens because of:

• network infiltration – where groundwater gets into old underground wastewater pipes
• inflow – where stormwater pipes are connected to the sewer network.

We're finding and fixing these problems as part of our everyday work.

Some parts of the wastewater network are getting old and need to be replaced and upgraded to support growth.

What we're doing

This upgrade involves laying 2.7 kilometres of new wastewater pipes. The new pipe will start at Riverbank Road, go along Aotaki Street and Mill Road, and end at Old State Highway 1. The pipe will be 800 millimetres wide on Aotaki Street and 500 millimetres wide on Mill Road. In some places it will be buried up to three metres deep.

We started construction in August 2024 and expect it will take up to 14 months. To make sure we complete the work as soon as possible, we have crews working in up to three locations along the route at the same time.

Moving wastewater

There are two main ways to move wastewater:

1. Gravity wastewater system

This system uses gravity to move wastewater downhill. The pipes slope downward from the house to the main pipe in the street, and then the street mains slope down towards the treatment plant.

Because the pipes keep getting deeper as they go downhill, it can become very difficult to lay or repair them. When this happens, a pump station is built with an underground tank to collect the wastewater. Big pumps then push the wastewater out of the tank and up to the next section of shallower gravity main, where it continues its journey to the treatment plant.

Wastewater pipes are usually the deepest pipes underground. The average depth of a gravity main is 3 metres.

Rising main

Pumps push the wastewater uphill through the pipes.