Whanganui's new $39 million wastewater treatment plant will be protected from excess industry waste under a proposed council bylaw.

The provision is part of Whanganui District Council's Proposed Trade Waste Bylaw which will open for public consultation next week.

"The main focus of this bylaw is to protect the new wastewater treatment plant and in doing so ensure that we don't get prosecuted for breaches of our resource consent," council's infrastructure manager Mark Hughes said.

Council's senior wastewater engineer Arno Benadie said a process protection unit would monitor industry waste.

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"Once they hit the limit of what the plant can achieve or can treat, this unit will unit will come on and remove whatever is in excess," he said.

"And the cost of the running of the machine, the removal of the material... will be paid for by the industries."

Mr Hughes said that was a more meaningful measure than allowing industries to breach limits and then fining them later.

"This is a trade waste bylaw so it only affects people that produce trade waste."

The new bylaw was written after consultation with industries and will replace the 2015 bylaw which was put in place as a "stop-gap" while the new plant is being constructed.

Meanwhile, how much industries will pay to discharge waste into the system will be determined as part of council's long term plan next year.

"This is more about controlling loads through to the plant and plant protection. We've got to delineate the two," council's finance manager Mike Fermor said.

The new bylaw needed to be in place by the time the new plant was running in the middle of 2018.

It will be available to consult on from December 15 at www.whanganui.govt.nz or at Whanganui District Council.