Police are urgently trying locate a man who reported being bitten by a snake south of Perth.

Nathan Scattini, 41, rang St John Ambulance about 6.40pm yesterday after being bitten but was not able to give them his location.

Police began searching bush in Kelmscott last night, where it is believed he had been walking.

The police helicopter, which is fitted with a heat-seeking infra-red camera, was used to scour bush but failed to find any trace of Mr Scattini.

The search has been continuing today.

Mr Scattini has a medium build, light skin and is about 175cm tall.

He is wearing a black top, shorts, sneakers and sunglasses.

Police are concerned for his welfare and say anyone who sees him should immediately call triple-0.