Repairs at Wellington's Moa Point treatment plant could take months, as raw sewage flows into the region's southern coastline.

An equipment failure caused the outfall pipe to become backed up with water - with last night's heavy rain flooding the building's lower floors.

Wellington Water says electrical equipment's submerged and some areas are inaccessible.

Chief executive Pat Dougherty says it's likely something bigger is wrong with the outflow pipe, but investigations are still ongoing.

"The real problem is the damage the water has done, it's huge amounts of wastewater poured through the plant. We've got control panels underwater, we've got electrical equipment underwater...and we've got no power."

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