Whanganui district councillors have been cautioned over any inquiry into the failed wastewater treatment plant.
The warning comes from chief executive Kym Fell following councillor Rob Vinsen's decision to put a motion calling for a full and open inquiry to the May 31 council meeting.
Mr Vinsen wants the inquiry to establish why the wastewater treatment plant failed, who was involved, and what was the decision-making process from its initial design to its final commissioning four years later in 2007.
He said the inquiry also needed to ask if council managed the plant satisfactorily and if it could have been salvaged instead of council needing to build a replacement at an estimated cost of $41.5 million.
Mr Vinsen reckons the inquiry could be concluded in three months and report back to a public meeting by August 31. He suggested the $75,000 in the draft plan for the cancelled online voting trial would pay for it.
Mr Fell said while he supported the notion of an independent inquiry, the terms of reference would need to be "very prescribed".
"It is important councillors carefully consider the associated costs and be very clear on the objectives of any inquiry. History cannot be changed but learnings can influence how we do business in the future," he said.
Mr Fell added that further delay of the construction of the new plant in order to get agreements with the trade waste industries would crystalise the risks outlined to council in March.
"These risks include extending the period in which untreated wastewater is discharged to sea with the resulting environmental, public health and reputational impacts, damaging the relationship with Horizons Regional Council and iwi, and council facing potential regulatory enforcement actions or central government intervention.
"It would also contravene the timeline given to the commissioners during the recent short term consent hearings," Mr Fell said.
Mayor Annette Main said Mr Vinsen's effort was unnecessary because "a motion lies on the table to look at what went wrong at the time".
The issues that hadn't been discussed at this stage were the scope of any inquiry and the cost of it.
"If there were faults in the decision-making at the time, is it likely this would be repeated? We've already seen the length of time it took council to make a final decision on the design of our replacement plant. I believe this was due to the desire to be fully convinced this design was the best for our community."
She said the council at the time accepted the views of the plant designers and staff that that was the best solution - an understandable decision "as councillors are not wastewater experts".
"What is important now is that we begin to build the plant we need in order to meet our obligations. Any review of earlier decision making has the potential to be extremely costly and time-consuming, and frankly what will it achieve?"