The source of a bad smell wafting around Palmerston North is under investigation (file photo).

A bad smell likened to sewage or silage wafting around Palmerston North since Wednesday evening has been identified.

Horizons Regional Council's pollution hotline had about 10 calls overnight about the stink.

On Thursday afternoon, Horizons spokeswoman Chrissie Morrison said the council's consents monitoring team had identified the source of the smell and those responsible for it had been asked to stop their activities immediately.

She was unable to give any further information about the source of the smell other than to say it was agricultural waste.

People complained about the offensive odour in the suburb of Milson, near Palmerston North Airport, and in The Square and central city.

Horizons is investigating.