Kaipara District Council says it is looking to the future with a proposed $68 million wastewater scheme upgrade for Mangawhai.

Just 136 connections are left before the controversial 13-year-old Mangawhai community wastewater scheme (MMWS) reaches its 3000-connection capacity, probably next year.

The $68 million spend would be spread over more than two decades and eventually boost scheme capacity by almost 70 per cent to 5000 connections.

Phase one will see $3.5 million spent almost immediately on installing technology to turbo charge wastewater processing capacity before December.

KDC general manager infrastructure services Anin Nama said the plant’s improvements would use technology similar to that Auckland’s Watercare was already using.

Nama said phase one would also involve turning the plant’s new holding tank into a third sewage treatment unit. It would also include more preparation for discharging the plant’s treated effluent on to the Mangawhai Golf Course via the sub-surface rather than an above ground irrigation option.

He said the goal was to use the golf course as the scheme’s main discharge option, rather than the Brown Rd farm which would shift to becoming a backup. The upgrade includes $15 million being spent on the Brown Rd farm the council owns.

Nama has put together a new multi-disciplinary team of experts in wastewater treatment, subsurface irrigation and construction cost estimation whose members have worked in New Zealand, Australia and across the Asia-Pacific.

The team had looked anew at what had initially been identified in a December 2022 master plan strategy requiring $90.5 million to be spent on the expansion over 10 years.

“The peer review has been conducted by leading experts in wastewater treatment and effluent reuse,” Nama said.

The review team had been asked to confirm the lowest cost pathway for MWWS to meet forecast demand growth and enhanced treated effluent disposal to the Mangawhai Golf Course.

Nama said he was aware of the scheme (MWWS)’s history. That was why one of the peer review team brought in was an expert on construction cost estimating, to ensure indicated figures were reliable.

The review team updated KDC on progress at a council meeting in Mangawhai on Wednesday. This included outlining the updated preferred option going forward and timelines.

Nama said a report based on their work would next be presented to the council in June. The council would then likely make a formal decision before the end of the year on proceeding with this phase.

When asked if Kaipara ratepayers would be paying for the new Mangawhai expansion Nama said the council would be looking at funding options for the first phase to deal immediately with addressing the scheme’s looming capacity limit. This could include borrowing.

The MWWS expansion comes on the cusp of a potential July 2024 transition to the new Three Waters governance and management system. This will see the helming of more than a $1 billion of Kaipara and Northland drinking water, wastewater and stormwater assets – and debt – shifted from three district councils to the new water service organisation known as Entity A. This will include Auckland Council’s council-controlled organisation Watercare.

Nama said necessary planning for funding and work on the project’s further phases has been done taking account of the transition to Entity A. Shifting to Entity A potentially means spreading the cost of Mangawhai’s $68 million extension across 1.8 million people, rather than KDC’s 25,000 strong population.

The expansion’s second phase includes $20 million to boost the wastewater treatment plant’s filtering performance to produce the A-grade treated effluent, and $7 million to set up the golf course for this effluent’s discharge. This phase is scheduled to begin in 2028.

The long and tortured history of the Mangawhai wastewater scheme has been mired in controversy since the need for a reticulated treatment plant became a significant issue for KDC in 1996, after septic tanks started polluting Mangawhai Harbour.

Mangawhai ratepayers were initially told the sewage scheme would cost no more than $10.8 million when it was announced in 2003. That went to $37 million when building started in 2009 and up to more than $60 million by 2013.

■ Local Democracy Reporting is Public Interest Journalism funded through NZ On Air.