The key is cost of the scheme, particularly the impost on residential ratepayers, if trade waste users don't join the scheme and at this stage that is still unresolved.
Mr Vinsen said no agreements had been signed with any of the wet industries - primarily Imlay, Land Meats, and Tasman Tanning - and this should be a major concern for residential ratepayers.
"All six councillors have agreed that if the wet industries sign up to the scheme - and that means their full agreement to pay their proper share of the costs - then we will all support the Cardno scheme as designed," he said.
"But if the industries fail to sign then they should be given two years to exit the council scheme, and the plant should be redesigned for the revised load."
He said plants can be scaled up or down, so if at any time in the future those industries wanted to be in the scheme then the costs for that should be the responsibility of the industry concerned.
Mr Vinsen said the elections delivered a clear message that there is an overwhelming concern amongst ratepayers that the WWTP was unaffordable for them.
"There's no confidence that the cost will not continue to escalate as it has done since it was originally approved by council in April 2013 at $18.93 million."
He said the Whanganui plant was unusual because 87 per cent of the biological load on the plant, and a major driver of both capital and operational costs, was from those wet industries and there was genuine concern that the residential sector would end up paying an unfair proportion of the total cost.
A week ago the six councillors wrote to Mayor Hamish McDouall and council chief executive Kym Fell saying that if any work had started on the Airport Rd site that it should stop until a review "determines an affordable way forward".
One of the opponents' arguments has been that the original lagoon-based process may well become a preferred option, and it would be unnecessarily costly to start work that didn't take this possibility into account.
But Mr Vinsen said by the end of last week Mr McDouall and Mr Fell hadn't given any reassurance, which left the six councilllors no option except to call for an extraordinary meeting.
Mayor Hamish McDouall said if a special council meeting was requested under Standing Orders "we will schedule it and furnish the new council with all the relevant information".
"The sooner the WWTP issue is resolved the sooner we can follow through the new council's mandate to support economic development in Whanganui."
Mr McDouall said nothing could happen until the new council was sworn in.