A group of Melton primary school students were on their lunch break when they saw a man with unzipped pants masturbating in a nearby playground, police say.

Police have released CCTV footage of the man loitering around the school, possibly with his hands down his pants, one week after the incident.

The Coburn Primary School students were sitting on a bench in the school grounds when they noticed a man in a nearby playground at Blackwood Drive Recreational Reserve, about 1pm on February 23, a police spokeswoman said.

"The man had unzipped his pants and was masturbating," she said.