Council staff will work through the holiday period to get the city's wastewater treatment right, says Mark Hughes, WDC infrastructure manager.

A stink has hung round Wanganui for more than a week following the dumping of material into the system from a still unknown source.

"What were trying to achieve is to get a steady state at the ponds, and we'll be working every day through Christmas and New Year to get it right," Mr Hughes said.

A week ago he said there was a thick layer of fatty scum floating on the top of the ponds and creating a foul smell. The scum was so thick it smashed the ultraviolet light tubes that provide the final treatment to the waste before it's pumped into the sea. Those lights have now been replaced.

"The fat floating on the surface of the ponds was going rancid. It baked in the sun. Even if the treatment plant was operating at its premium, it could never have coped with that dumping of extra waste into the system," he said.

The city's wet industries told council what loads they were going to put into the treatment system during the year, and three or four times a year council staff monitored those industries on site.

Mr Hughes said council just doesn't know which of the industries was responsible for pumping the abnormally high levels of protein into the system.

But he said council would work with those industries to implement a new regime of random testing.

"The point is we can't guarantee this sort of thing won't happen again, but we want to be checking more often and randomly and will be meeting with those industries in the new year," he said.

He said closing the treatment plant for a period was not an option.

"We could divert the untreated waste directly through the South Beach outfall into the sea, but that's just not an option."

As the stench intensified, council staff have been dosing the ponds with odour-treating chemicals as well as spraying the pond surfaces each day.

The aim of the chemicals targeting the stench is to improve the oxygen levels in the top metre or so of the ponds before Christmas. Similar treatment is happening at the Beach Rd pumping station before the waste reaches the ponds.

Mr Hughes said the extra processes added to the ponds were not impeding the bio-augmentation (bacterial) process already in place. He said gains were being made until the unexpected waste was dumped into the system.

Yesterday Nic Peet, Horizons' group manager of strategy and regulation, visited the ponds with Mayor Annette Main, and said his council was supportive of the approaches the WDC had taken.

"The formal warning from us puts the council on notice but we're comfortable with what it's doing to resolve this problem," Mr Peet said.

- WANGANUI CHRONICLE