Cause of unsightly Wellington wastewater blockage explained

Wellington Water says a blocked wastewater pipe packed with fat, oil, grease and wet wipes caused Saturday's wastewater overflow onto State Highway 2 and into Wellington Harbour.

Crews spent most of the day locating the blockage, which caused a manhole cover to burst open.

Wellington Water chief operating officer Charles Barker said the overflow discharged an estimated 55-110 litres of diluted wastewater a second into the harbour through a stormwater outlet, while only a small amount reached the highway itself.

He said the overflow was not caused by ageing infrastructure.

"No wastewater infrastructure is designed to handle people putting the wrong things down the toilet," he said. "In this example, it's not the age of the asset, it's not the lack of maintenance, it's simply people putting rags, wet wipes, things which are not meant to go down the toilet down the toilet, resulting in a blockage."

Barker said the spill had now been fixed, the road repaired and the area disinfected.

Meanwhile, Wellington residents were reminded to stay off south coast beaches, following the latest discharges from the Moa Point treatment plant.

Wellington Water said the plant had pumped unscreened and untreated wastewater into Tarakena Bay from 8am to just before 9.30pm Thursday, and again from 9am Friday until 7am Saturday.

"It's important to note that this was a capacity limit," a spokesperson said. "If the treatment plant cannot process all the incoming flow, wastewater may overflow from parts of the network to prevent flooding of homes.

"In this instance, the long outfall pipe reached its limit and the excess had to be overflowed to the short outfall."

Barker said the latest discharges came after inflows surged from about 810 litres a second on a normal day to roughly 3000 litres a second.

The treatment plant had relied on the short outfall since it's catastrophic failure in February, because there was a limit to how much wastewater could be sent through the long outfall.

Barker said people should continue following Land, Air, Water Aotearoa's advice to avoid swimming for 48 hours after heavy rain, regardless of wastewater discharges.

He said recovery of the plant remained on schedule and it was expected to restart in November.

"Treatment capacity will then improve over the following 6-8 weeks, as the biological treatment process returns to full strength."

Barker said software changes meant the type of failure that flooded the plant in February could not happen again.