Persistent debate about the city's wastewater treatment plant is destabilising growth and deterring potential new business development in Whanganui, according to one district councillor and mayoral aspirant.

Councillor Helen Craig said a sense of unreality "and quite frankly nonsense" was entering the debate on both the failed WWTP and new plant.

Mrs Craig said she could understand the community anxiety but council commissioned a new plant based on the only decision possible.

"We've done our job after an exhaustive process. But passionate people who believe they know more about wastewater plants than three independent contracted experts undermine confidence to the point they could destabilise our current growth phase and put off new business investment."

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She said the legal settlement council reached against the old plant designers MWH meant council could now begin a transparent and an open independent inquiry into the failure of the old plant without compromising that settlement.

"The confidentiality clause is extremely unpopular but that's the terms of the settlement our professional legal counsel could negotiate. We all wish this could be made public as there's no advantage to council for continuing confidentiality."

Mrs Craig said independent and expert advice determined the design of the original plant was extremely experimental and would never have worked.

She said the new plant was designed by the best in the world, peer reviewed by two independent experts and was similar to a proven design currently operating in Lower Hutt.

Urgency was needed because Horizons will not allow council to keep pumping raw sewage and industrial waste through the South Beach ocean outfall for any longer than three years, the window council had to build this new plant.

Mrs Craig said based on estimates of running costs in three years' time, the city's major industrial users can then estimate their disposal costs and determine the cost benefits to build more pre-treatment at their own sites.

"But until all that happens, no predetermination of final rates allocation could be made.

"No one wants to pay increased rates, so the longer the new plant build is delayed, is advantageous to them.

"However, raw sewage and industrial chemicals going out to our coast are a risk to our health from contaminated seafood and beaches.

"Let's stop the nonsense repeatedly visiting decisions and get on with growing jobs and population."