Auckland oyster farmers are furious after what is believed to be the biggest wastewater spill into the Mahurangi River this year.

Ten marine farms have been in a lengthy battle with Watercare to stop sewage overflow contaminating their oyster crops.

Mahurangi Oysters owner Jim Aitken told RNZ that, in an email on Thursday, Watercare informed them of an overflow at its Lucy Moore Memorial Park pump station.

In the email, seen by RNZ, Watercare said at about 2.30pm the pump station overflowed to the stormwater pond and wastewater began entering the Mahurangi River.

"We believe the overflow was caused by a power surge that tripped our pumps.

"Unfortunately, this was not flagged by our internal alert system, and we did not discover the overflow until a site visit this morning. The pump station was reactivated, and the overflow stopped about 8am [on Thursday]."

Watercare estimated the overflow volume was about 1200 cubic metres, but did not know how much had reached the harbour.

Aitken said continuous pollution of the water had ruined his business.

"We're going under, simple as that. I'm now burning through my personal cash flow. I actually advertised this week for another staff member to help us out over Christmas and next year. I've pulled the pin on the whole thing, I can't afford it.

"It's heartbreaking to see New Zealand, once known for clean, green food production, become unsafe because of repeated spills by Watercare.

"The future we've invested in for our family's legacy and our retirement is going down the drain. Watercare needs to be held to account."

Aitken said the family-owned farm had to close for 28 days because of this latest spill.

He said it had 80,000 dozen oysters ready to be sold, many of which would now have to be dumped.

Mahurangi Oyster Association chairperson, Lynette Dunn told Morning Report the sewage overflow comes at the peak of their season.

"We need financial support from Watercare as this is going to continue happening for the next two years until the new treatment plant is up and running."

Dunn said issues regarding sewage had been a problem oyster farmers in the area had been battling for seven years.

"So we are just pretty worn out about the whole thing, just the whole non caring about the harbour itself as well."

She said it can create dead zones in the harbour where marine life can't survive.

Dunn said there was not one business in the harbour that was not struggling.

'Extremely disappointed' - Watercare

In a statement, Watercare chief executive Jamie Sinclair confirmed the "significant" wastewater overflow and said the company had alerted Auckland Council's pollution response team and the Ministry for Primary Industries.

"We are extremely disappointed and sorry that this has happened, particularly for the impact it is having on the local oyster farmers, who now face a 28-day harvest closure," Sinclair said.

"We had been seeing a positive impact from our recent improvements to the Warkworth wastewater network that had allowed oyster harvesting to resume, so we know this news will be very upsetting to farmers who are nearing the end of their harvest season.

"I have been in touch with the oyster farmers to convey how disappointed we are, and hope to meet them soon so I can apologise in person on behalf of Watercare and offer our support."

The pump station did have internal storage, but this had overflowed to the nearby stormwater pond, and wastewater began entering the river.

Crews had pumped the combined wastewater and stormwater from the pond back into the pump station, which was operating normally, on Thursday afternoon, Sinclair said.

"We have begun an investigation into what caused the power surge, and to look into why our alarm systems and monitoring didn't work as we'd expect. We are determined to learn from this and ensure we don't face a similar incident in the future."

This story was originally published by RNZ