Whanganui District Council has settled its legal action against global engineering consultant MWH over its work on the wastewater treatment plant.

But details of the settlement will not be made public.

A mediation process was finished in November and the two parties have agreed to settle the claim the council brought against MWH over the design of the plagued treatment plant.

"The parties have agreed to this settlement in order to bring to an end the ongoing uncertainty and the need to fund and participate in a trial that would otherwise take place towards the end of this year," council chief executive Kym Fell said yesterday.

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The terms of the settlement are confidential.

The council is to replace the plant at an estimated cost of $38 million - though that may rise.

The council decided in July 2013 to sue MWH over the plant, which has been troublesome since it was commissioned in 2007.

It cited shortcomings in the concept and final design of the Airport Road facility, though MWH has always denied it was at fault.

Early reports suggested the council was seeking up to $10 million in compensation.

Up until the end of December, the council had paid its lawyers $860,000.