Untreated water is leaking onto the capital's south coast beaches due to the Moa Point Treatment Plant flooding and being turned off from early today due to a mechanical failure.
Fire and Emergency NZ shift manager Murray Dunbar said three fire trucks responded to an alarm activation about 12.40am.
Crews reported they were unable to access the lower building due to flooding and waited for a building representative to be contacted and respond, Dunbar said.
The plant was shut down and staff evacuated as multiple floors were flooded.
Wellington Water chief executive Pat Dougherty told media today it could take months to fully repair the sewage infrastructure.
"This is a serious situation and we anticipate the plant will be shut down for an extended period."
Work was being done to replace UV lamps at the plant which was where the wastewater backed up, but Wellington Water still did not understand why, so further work would be done on that, he said.
A room at the bottom of plant, the size of an Olympic swimming pool, was 3 metres deep in wastewater.
Warning for south coast beaches
Dougherty strongly advised the public to stay away from south coast beaches, and said a rāhui (ban) would be placed over the affected area.
"We will have boots on the ground today, with our customer teams distributing information about public health and advice.
"An environmental team will be undertaking water quality testing. More information will be provided at lunchtime today. This is a complex incident, and all necessary resources are being utilised.
Dougherty said the situation was unacceptable.
"We apologise to the public, local community and our iwi partners for the public health and environmental impact caused."
The first goal was to redirect wastewater out to the long outfall in the Cook Strait, the second priority was to get sludge out of the treatment plant, thirdly to get wastewater out of the plant and to see how badly damaged it was, because the plant had no power there was no ventilation, and workers had to be very careful due to dangerous gases, he said.
Wellington Water would be doing an investigation into what went wrong and whether it could have been prevented, and would report back to the public, Dougherty said.
He said they needed power to send the wastewater to the long outfall in Cook Strait.
The biggest impact of the event was going to be on Wellingtonians being denied the use of the south coast for the next few weeks, if not months, he said.
It would take a few days before they knew how much equipment had been lost and what the cost of that might be.
"We will be applying all our resources to getting that long outfall going as quickly as possible."
Dougherty said the outfall pipeline had more capacity than the treatment plant itself, and he was at a loss as to why it backed up there.
Incredibly disappointing - mayor
Wellington Mayor Andrew Little said it was a "hugely significant event, obviously incredibly disappointing".
People would not be able to go on the beach for the next few days and this could not have come at a worse time, he said.
In terms of the Island Bay festival, scheduled for this weekend, Little said he did not know what it would mean for the blessing of the boats ceremony.
He said people should plan not to swim, go on to the beach, let dogs or kids onto the beach as the risks were too high.
About a dozen people were walking their dogs along Lyall Bay Beach this morning.
Diana told RNZ she saw signage advising people not to swim or take seafood because of the pollution.
Maggie said the wastewater discharge was a big concern and she likely wouldn't swim in the area for a while.
"Because who wants to swim in crap?"
Sally said usually the beach was packed around lunchtime, but today there were not many people. She supported seeing more investment in Wellington's water infrastructure.
"I mean, we're the capital city of New Zealand, so you'd expect we'd have kind of world class services."
Tom was also keen to see more water investment if it meant wastewater wasn't dumped into the ocean preventing people from being able to enjoy the beach.
Taranaki Whānui chief executive Kara Puketapu-Dentice told RNZ the situation was unacceptable.
"It is like we have got Third World infrastructure in a First World country and our environment is suffering because of that."
Puketapu-Dentice said the city needed to come up with some solution quickly and the iwi would be following closely any delveopments and reviews of the plant failure.
The Moa Point problem follows two other wastewater discharge notices on Tuesday night, which were caused by heavy rain.
Partially treated wastewater was discharged into Wellington's Karori Stream at 10.45pm, which flows into the sea on the south coast.
Before that, fully treated sewage was released at 10.18pm into Waiwhetū Stream in Lower Hutt, which flows into the sea near Petone beach.
Water monitoring body Land, Air, Water Aotearoa advised people to stay out of the sea for two or three days after heavy rain.
