The Wanganui District Council has been issued with an abatement notice, over a foul odour that has afflicted parts of the city since before Christmas.
The Manawatu-Wanganui Regional Council says the district council has broken the Regional Air Plan.
The district council says the smell is partly caused by sludge that factories are discharging into the city's wastewater system, which has failed to cope.
The Regional Air Plan states no objectionable odours should be able to be smelt beyond the waste treatment plant's boundaries.
Mayor Annette Main says she has spoken individually to factories open over the Christmas period, and has also sent letters reminding them of their consent obligations.
Ms Main says it is impossible to identify where the extra waste came from so no-one will be held accountable.
She says the smell appears to be more under control and complaints have reduced.