Greater Wellington Regional Council is prosecuting Wellington City Council and Tiaki Wai over the catastrophic failure at Moa Point wastewater treatment plant.

Billions of litres of raw sewage have spilled into the city's south coast since the plant flooded in February this year. 

Wellington City Council says it anticipated the prosecution.

It acknowledges the seriousness of the impact on the environment, residents and businesses on Wellington's south coast.

The council says it's co-operating fully.

Greater Wellington said Tiaki Wai and the city council could face tens of millions in fines if convicted.

In a statement, it said each organisation faced 10 charges related to what it claims are unlawful sewage discharges from the plant.

The charges covered two periods: between October 2025 and January 2026 when the plant's ultraviolet system was being upgraded, and the failure at the plant in early February this year and ongoing discharges of sewage connected to it.

Greater Wellington director strategy, policy and regulation Fathima Iftikar said the decision to prosecute followed a detailed investigation into events, the roles of the parties involved and the environmental consequences.

It was not taken lightly, she said.

"As an independent regulator, Greater Wellington is responsible for enforcing compliance with the Resource Management Act (RMA), including compliance with resource consents and the regional plan.”

Each party faced 10 charges: five related to what Greater Wellington said were unlawful discharges, and five for breaches of an abatement notice.

Under the RMA, if convicted Tiaki Wai and Wellington City Council each faced a maximum fine of $10 million per charge, Iftikar said, however that would be up to the court.

"No matter how important the service, or who owns or operates it, the obligation to comply with environmental law remains the same."

Iftikar said Greater Wellington was separately considering if regulatory measures were needed to ensure the plant returned to full compliance.

Work to restore the plant is ongoing.

Porirua Mayor Anita Baker said there were no winners in Greater Wellington Regional Council's prosecution of Wellington City Council and Tiaki Wai. She said the two sides should have tried to work out a solution without going to court.

"With water at the moment, we're needing money in pipes, not money in the lawyers - and it's all ratepayer money, no matter which way you look at it."

The environmental damage had already been done, and the focus should be on making sure it did not happen again, she said.