Former Whanganui mayor Michael Laws says he will contribute "any and all" the information that was available to him to the pending inquiry into the failure of the city's wastewater treatment plant.

Mr Laws was the district's mayor for two terms from 2004 to 2010. The Airport Rd plant was commissioned in 2007 but shut down in 2012.

Yesterday he said the council's decision to proceed with an independent inquiry into the failure of the plant was "long overdue but completely necessary".

"I look forward to being contacted by whoever is tasked with the inquiry, and will contribute any and all information that was available to me at the time," he said.

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"I do need to publicly state that, in contradiction of Cr Rob Vinsen's unsubstantiated allegations, there was no political pressure placed by myself or my governance team upon the council's management to save costs over the treatment plant."

Mr Laws said the council had inherited what he considered to be a "disastrous" $120 million wastewater separation policy and the projected cost of the plant itself was minor in comparison.

He said it was worth noting that the original design was peer reviewed by three external expert agencies.

"One question that the inquiry will need to address is whether the then council received all or sufficient information from its management team. Certainly there was no indication provided to council of the day of any undue risks.

"To this day - and because of this the MWH (the plant's designer) evidence remains critical - I still don't know whether the design was actually at fault or (if it was) the way that the council managed the plant."

But Mr Laws said unless an inquiry addresses that issue and has access to all the information, including the MWH files, it cannot properly reach a conclusion.

"To proceed without all the available information would be to throw good money after bad.

"We all want the truth. But an inquiry won't get the truth when it only hears engineering evidence from the party - that is the district council - that has a vested interest in exonerating its own actions," he said.

Council is expected to name the person who will head the inquiry tomorrow. It's expected the inquiry will take about three months and cost about $100,000.