Repairs to Wellington's Moa Point Wastewater Treatment Plant could take months after a sewage back-up flooded multiple floors and caused an ongoing raw sewage discharge that has closed beaches off the capital's south coast.
Speaking from Lower Hutt, Wellington Water chief executive Pat Dougherty said heavy rain had caused wastewater to back up into a maintenance area and flood multiple lower floors of the facility, tripping power fuses, damaging control equipment and forcing power to be switched off.
A scrubber room "the size of an Olympic swimming pool" was now three metres deep in wastewater, he said.
At the peak of the rain event, flows reached 3300L per second, although Dougherty noted this was largely stormwater.
Wellington Water was prioritising diverting discharge to the 1.8km long outfall into Cook Strait in the coming days once power could be restored, removing sludge to prevent odours, and assessing the extent of equipment damage — which could take months to repair, Dougherty said.
He said he was "at a loss" to understand why the long outflow pipe backed up.
"The outfall pipeline has more capacity than the treatment plant itself, it's designed to to be the one thing that works whatever happens."
He strongly advised the community to stay away from beaches on the capital's south coast.
"While we're using that short outfall, I think the message should be, stay away from the water. We will revisit that once we can change to the long outfall, it's further away from the coast, and there's better dilution, but we don't know yet."
Public health signage was being put up at Lyall Bay and along the south coast beaches to warn of the risk, with a rāhui to be placed in affected areas and daily environmental testing to take place.
Dougherty said the incident was "unacceptable".
"We apologise to the public, local community and our iwi partners for the public health and environmental impact caused."
Wellington Mayor Andrew Little said the event was "hugely significant" and "incredibly disappointing", especially at this time of year.
"This could not have come at a worse time," he said.
"This is the time of the year when people go to the beach, both for swimming, but also for just getting down to the beach, walking their dogs on the beach. People won't be able to do that for the next few days at least."
Greater Wellington Regional Council issued a warning on the Land Air Water Aotearoa website for Ōwhiro Bay, Island Bay, Princess Bay and Lyall Bay.
"We strongly advise that people avoid the coastal area along the south of Wellington until further notice. Do not enter the water or collect kaimoana from this area."
A temporary discharge notification posted at Tarakina Bay said swimming and aquatic activities were not recommended for 48 hours.
The collection and consumption of shellfish is not advised at any time, it says.
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