Wanganui stank more on Saturday because a load of fat was sent into the wastewater system.
The fat later floated to the top of the wastewater lagoon and turned rancid.
The Wanganui District Council does not know which industry was responsible.
Mayor Annette Main said it was "imperative industries cease discharging more than their allowed amount of trade waste into Wanganui's wastewater system".
She also revealed that the plant has not worked properly since it was opened in 2007.
"Our existing problems with the wastewater treatment plant have been made so much worse by an industry, or industries, discharging unexpected large amounts of trade waste," Ms Main said.
"We have agreements with eight industries which specify the amounts of trade waste they can put into the system. Since just before Christmas, there have been a number of instances where large amounts of trade waste have been discharged, greatly increasing the odour problems at different times in various areas of Wanganui."
She said only one company, "to their credit" had informed the council of a recent discharge above agreed levels.
The Chronicle understands this is Tasman Tanning and the council was notified in advance of the discharge.
Mrs Main said while the public may believe the council must know who has discharged excess waste, the system is not set up to be able to identify where the discharges come from.
"While we are putting our time and resources into continuing to reduce the smell problems, the long-standing inherent issues with the plant itself need to be addressed. It is obvious that the plant has not been working properly since it was opened, and that needs to be sorted."
Wanganui District Council infrastructure manager Mark Hughes said that, following the discharge of fat on Saturday, "sucker trucks were brought in to remove the fat and the dosage from the new atmospheric odour neutralisation system was increased".
"This system, which was installed over parts of the plant late last week, sprays a fine mist into the air around the plant. However, the system could not cope with the increased odour, so it will be extended to cover the whole plant. " Provided that materials arrive from the US in time, installation of the additional atomisers will begin tomorrow.