A pizza deliver driver has been robbed of cash and pizzas by a man with a 30 centimetre knife in Wanniassa overnight.

The incident happened about 9.30pm on Thursday, when the driver was sitting in his car in Byrne Street.

A man wearing a black balaclava or cloth over his face appeared in front of him and held a 30 centimetre knife to his stomach, demanding money.

After the delivery driver handed over a small amount of money and a number of Hot Cell pizza boxes with pizzas and garlic bread inside, the offender fled on foot along a bike path next to Byrne Street. The driver was not injured.

The offender was described as about 22 years old, 170 centimetres tall and Caucasian in appearance, police said.

Police ask anyone with information that could help identifying the man, or anyone who witnessed any suspicious activity in Wanniassa last night, to contact Crime Stoppers on 1800 333 000, or via www.act.crimestoppers.com.au. Information can be provided anonymously.