A brighter picture is being painted of the overall operating costs of Whanganui's replacement wastewater treatment plant than initially forecast.

Whanganui District Council chief executive Kym Fell said council staff have refined the operating costs of the Airport Rd plant and this, coupled with low interest rates and foreign exchange movement, has produced a "more favourable" cost than that forecast in the 2015-25 10-Year Plan.

The changes affect the operating costs of the plant and not the capital costs of building the new plant which remain that same at $41.5 million.

Mr Fell said the continuing review aims to reduce costs to residential and commercial properties, as well as to the trade waste industries.

Advertisement

"We've been doing a lot of analysis behind the scenes on our financial modelling to see how we can make the plant more affordable for our community," he said.

He said calculations were made on the basis that all current trade waste users of the scheme will continue to be part of the new scheme.

"Affco and its associated companies have not formally opted out, leaving us no choice but to include them, in accordance with the Local Government Act.

"Firming up sound trade waste principles will be our focus over the next three months to form the basis of our future discussions with trade waste users."

Mr Fell said based on the amended financial forecasts pan taxes to individual properties will increase about $2.38 per week which is significantly less than the original forecast of a $4.79 increase.

And he said trade waste charges would total about $2.8 million annually rather than the $4.5 million proposed in the 10-Year Plan.

He said while the changes to the operating costs represented an "affordable outcome" for the community, the amended figures will still be subject to the council striking the rates and any change will not take effect until the new treatment plant is up and running.