The Wanganui District Council says it will keep testing over the coming days to gauge the success of its attempts to reduce a foul odour.
The stench has been afflicting parts of the city since before Christmas.
The council says the smell is partly caused by sludge made up of discharges from factories into the North Island city's wastewater system, which has failed to cope.
Mayor Annette Main says effluent from stock trucks and wet compost has also contributed to the problem.
On Thursday, work began on a new odour neutralisation system and the second part of that project was completed on Friday night.
The council's infrastructure manager, Mark Hughes, says a fine mist is being sprayed into the air at 30 different locations, and Ms Main says that seems to be helping.
Mr Hughes says over the next few days the council will work to find the most effective concentration mix and spraying regime.