The Moa Point Treatment Plant. Photo: RNZ / Samuel Rillstone

Wellington Water says it may have missed early warning signs that there were troubles with the discharge at Wellington's wastewater treatment plant.

It comes as leaders, including the prime minister, are backing calls for a governmental inquiry into Moa Point's Wastewater Treatment Plant fiasco.

Crews have spent the weekend trying to clean up Wellington's wastewater plant so they can assess the damage after it was shut down due to being flooded by raw sewage. It's likely to be months before the Moa Point plant is back in operation.

In the meantime, millions of litres of untreated sewage continue to spill out into the Cook Strait since early Wednesday.

Wellington Water chief executive Pat Dougherty. Photo: Samuel Rillstone

Wellington Water chief executive Pat Dougherty told Morning Report people may have to stay out of the water around Wellington's south coast for months.

Dougherty said Monday was the first day they could get staff safely into the building to assess the damage.

"From the limited inspections we've done, we think 80 percent of our electrical gear has been either inundated or partially inundated. So, the damage is significant," he said.

He believes the issue could be where the pipe bypass combines with the discharge from the plant.

Dougherty said there's been under-investment over a long period at the Moa Point plant and backs an investigation.

"I worry that there may have been some early warning signs that there were troubles with the discharge and we missed those. But everything needs to be on the table," he said.

He said there have been a couple of incidents over the last few months that he suspects may have been early indicators.

Prime Minister Christopher Luxon. Photo: RNZ / Samuel Rillstone

Prime Minister Christopher Luxon told Morning Report the failure was "catastrophic" and he would be further talking with the mayor later on Monday.

"It is incredibly concerning for the public, and I can understand their frustration about the quality of their water assets in Wellington in particular," Luxon said.

He said he would support a government inquiry.

"It is important that we do get to the bottom of what has actually caused such a failure in this way."

Green MP Julie Anne Genter, who is the local MP, told Morning Report earlier on Monday the plant's failure was devastating.

"It's really disappointing. I know so many people in Wellington love our south coast, and this is precisely the time they would be out there on a beautiful day on the beach," Genter said.

"… It's bad for us, but it is even worse for our wildlife."

She said there were little blue penguins, or kororā, in the bay and a pod of dolphins there on Sunday.

"We can do better, we have to do better."

Green MP Julie Anne Genter. Photo: RNZ / Angus Dreaver

Genter said the council was putting enormous amounts of money into Moa Point, with an upgrade happening at the wastewater treatment plant.

She said concerns have previously been raised over the performance of Moa Point plant's privately-owned operator Veolia.

"There seems to be some separation between the private company, Wellington Water and the ability of the elected people to get the results that we need," she said.

Genter said there needs to be a full investigation.

Wellington mayor Andrew Little said last week that he would raise the "catastrophic" failure of the city's sewage plant with the prime minister on Monday.

Little said one of the questions in the days ahead would be about setting up an appropriate investigation or inquiry into what happened.

"We can't let this happen again," he said. "We can't let such a critical plant for a modern city fail in the way that this has and cause the environmental degradation that it has.

"It's important we get the facts and it's important we understand what we need to do to prevent it from happening again."

Dolphins have been spotted along the south coast over the weekend and there were several in Lyall Bay on Monday morning.

David Lundquist, Department of Conservation senior science advisor in the marine species team, said they were aware of reports of dolphins in the area.

"It is common for dolphins to be seen off the Wellington coast over summer, so their presence there is not unexpected.

"At this stage, there is no sign the dolphins are in distress due to the spill."

He asked the public to call DOC if they saw any unusual activity or dolphins in distress.

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The smell from the massive Moa Point raw sewage discharge in Wellington could be about to get worse, as Wellington Water battles a new overflow in Porirua.

An average of around 70 million litres of untreated wastewater has been pouring into Wellington's South Coast each day since Wednesday morning. Audio

The leak may impact sponges, muscles, penguins and fish that eat particles in the water. Audio

Meanwhile, Wellington Water said screened wastewater was now discharging to the long outfall pipe again.

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