Central Interceptor extension proposed to clean up city fringe beaches

We're planning to extend the Central Interceptor wastewater tunnel all the way to Pt Erin in a bid to clean up Auckland’s city fringe beaches at St Marys and Herne bays.
Our team and Auckland Council’s Healthy Waters had originally planned a programme of work to separate the combined stormwater/wastewater network in St Marys Bay and Herne Bay, as part of the Western Isthmus Water Quality Improvement Programme, which would significantly reduce overflows into the harbour when it rains.
However, our chief executive Jon Lamonte says recent cost estimates have prompted a rethink on the best approach to achieving the water quality outcomes the community is eagerly anticipating.
“Infrastructure projects all over the country have been heavily impacted by escalating material, labour and traffic management costs. These increases, along with further costs identified in the design stage to accommodate geotechnical risks, put our most recent cost estimates for the St Marys and Herne Bay separation project at $278 million – well above the original forecast of $136m.
“We simply can’t continue with the original separation plan at that cost so we’re developing an alternative solution that involves extending the Central Interceptor wastewater tunnel that’s currently in construction.
“By extending it about 1.5 kilometres from Grey Lynn to Pt Erin, and building a new Herne Bay wastewater pipe to connect to it, we’ll be able to collect the area’s combined wastewater and stormwater and transport it to our Māngere Wastewater Treatment Plant.
“The Central Interceptor extension is an excellent alternative as it will achieve the same environmental benefits but sooner and at about 20 per cent less cost than separation.
“We’d be almost eliminating overflows at these city beaches by 2028.
“It would also mean there’d be less disruption in the St Marys and Herne Bay communities from construction activity and traffic management as the tunnel boring machine plugs away underground, unnoticed, and construction activity would be localised to the Pt Erin drop shaft.”
Together with Healthy Waters, we are engaging with local community liaison groups and mana whenua about the alternative approach.
While the separation programme won’t proceed in the 2028 timeframe originally intended, separation will still be carried out in the future using a targeted approach to gradually remove stormwater from the Central Interceptor tunnel.
Our team and Healthy Waters staff are hosting a drop-in session at the Ponsonby Community Hall on October 12, from 6pm to 8pm to answer any questions residents may have.
We hope to progress with detailed design and statutory approvals for the Central Interceptor extension later this year.
Extending the tunnel to Pt Erin is expected to add another three or four months to Central Interceptor project, which is currently scheduled to finish in mid 2026.