The likely cause of Wellington’s sewage disaster has been revealed, with a report finding trapped air in the system may have caused millions of litres of raw, untreated waste to be pumped into the city’s coast. 

A hydraulic report commissioned by Wellington Water found air can become trapped in the wastewater bypass pipeline and disrupt the flow of wastewater, pushing it backwards and flooding the plant. 

“While this is valuable information, these findings are just one piece of the puzzle. There are other areas of interest that require a more detailed investigation,” Wellington Mayor Andrew Little said. 

The failure is currently at the centre of a Government-ordered investigation. 

Raw sewage continues to be pumped out to the Cook Strait from the Moa Point plant’s long outfall pipe near the mouth of Lyall Bay following the total plant failure in early February. 

The “catastrophic failure” happened while remedial work was being carried out, Wellington Water said at the time. 

The Moa Point Wastewater Treatment Plant yesterday suffered a complete failure, causing raw sewage to flood the plant and flow freely out into the capital’s South Coast beaches. 

Wellington Water chief executive Pat Dougherty estimated about “60-70% of the plant was inundated” with wastewater when it happened, including control and electrical equipment, while untreated waste was pumped out of a pipe just metres off the coast. 

The report into the incident developed a hydraulic model of the effluent pipelines, which carry wastewater out of the plant, the bypass system, which allows screened wastewater to be discharged out the long outfall pipe when the plant is at maximum capacity, and the long outfall pipe. 

The report hadn’t been commissioned to identify the cause but had revealed the “valuable” information. 

“It’s natural to want a quick answer, but it’s important that we take the time to uncover all the facts and not draw conclusions too early,” Little said. 

“It’s crucial we keep an open mind while the other reports and findings of the Crown Review Team are prepared. We’ll continue to be fully transparent with the public, and all reports will be released at the appropriate time.” 

More to come. 

Ethan Manera is a Wellington-based journalist covering Wellington issues, local politics and business in the capital. He can be emailed at [email protected].