Whanganui's deputy mayor has dismissed claims that the district council has been "evading" a proper consultation process around the city's new wastewater treatment plant.

The allegation was one of several made in an open letter to councillors and the community, and published in last Friday's Chronicle.

It was signed by a group calling themselves "concerned business ratepayers" and included councillor Charlie Anderson, David Bennett, boss of Pacific Safety, mayoral candidate Andy Jarden, Ratepayers Association chairman Graham Adams, rural community board member Alan Taylor, Neville Johnson of Financial Progress and Midwest Ferries, management consultant Bernard Corkery and civil engineer Graeme Young.

They were concerned the council would sign up for a new treatment plant "based on a seriously flawed decision-making process".

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They asked whether, bearing in mind the magnitude of the debt being taken on with the $41.5 million plant, the council had been evading its duty to consult. But Hamish McDouall said that was not the case.

"The council has fully consulted during the recent 10-year plan amendment and 2016-17 annual plan process, and only 25 per cent of submitters disagreed with the approach.

"Forty-four per cent either agreed or strongly agreed with the approach and some of these submitters (44 per cent) represented the views of the majority of businesses in Whanganui.

"Indeed, some of the signatories to the 'open letter' took the opportunity to submit to council," he said.

The signatories had also questioned the legality of the tender process and if the council was sure this was the best possible price. Mr McDouall said the council had checked the legality of its process and was assured it was lawful within the Local Government Act.

"Six contractors were invited to tender on price (and) the best price led to the selection of the preferred tenderer," Mr McDouall said.

"Increases since then have only been for additions to the design - a lump sum for the dryer and increases due to inflation and foreign exchange movement during the protracted period of delay by council."

Asked if the design could be modified, he said the design was completed and was being modified to incorporate the dryer: "Further modifications are possible but would have time and cost implications."

The open letter also asked why the "wet industries" had not been contractually bound before a tender was accepted.

Mr McDouall said discussions were continuing with those industries but the council would set trade waste and pan tax rates when the new plant was operational in two years' time.

"Contractually binding prices cannot be set at the present time. Two industries have written to us to raise concerns over possible pricing and this is subject to ongoing discussions," the deputy mayor said.

The council was also asked if Horizons Regional Council had imposed higher effluent outflow requirements than councils in Hawke's Bay were required to meet. Mr McDouall said the effluent outflow requirements were the same as they were for the failed plant.