Whanganui District Council says it will tighten checks on companies putting waste into the wastewater treatment plant after excessive dumping caused a foul odour to waft into the city.
Mayor Annette Main said the plant, which was built in 2007, has a design fault which means it cannot handle the quantity of waste produced in Whanganui.
The plant uses two ponds at the moment, but two more need to be added.
Ms Main said there has never been a formal logging system to check who is putting what into the plant at what times.
She said that over time, companies have put in more than they are allowed and she wants that changed so the council know how the plant is being used.