Whanganui District Council will meet in special session on Monday, July 4, to discuss what form an inquiry into the city's failed wastewater treatment plant will take.

Last month councillors debated whether an inquiry would be held and, once over that hurdle - it took a couple of hours of discussion to reach the point where the majority of councillors favoured an inquiry - it became a matter of what form it would take.

The July 4 meeting will focus on who will head it, what it will seek to answer and how much it is likely to cost.

The call for an inquiry was a motion on the council books since June 2013 before it was overtaken by the legal action against plant designer MWH. It was back on the agenda on May 31 with council also deciding the workshop would be open to the public.

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The Wanganui Ratepayers' Association presented a 1023-signature petition asking council to initiate an independent inquiry and suspend any spending on the new $42 million plant until the outcome of the inquiry was known.

Earlier council chief executive Kym Fell had been asked to prepare a budget and timeline for an inquiry and he said given the complexity of an inquiry costs could be up to $300,000.

Councillors were at odds over what the inquiry would be expected to achieve, but they were adamant it was not about naming or blaming people who may have been involved.