The battle for the seat of Higgins has spilled into violence with police investigating the alleged biting of a Greens volunteer by a backer of assistant treasurer Kelly O'Dwyer.

Investigators have been told a 48-year-old woman was sitting in a car with a friend outside Armadale Public School on Densham Road about 9pm on Friday when two men - aged in their 60s - approached her vehicle.

The men allegedly made comments about election material for the hotly contested seat of Higgins, where a "green army" of volunteers spent the night capturing the best spots for election posters, before the woman attempted to take their photo.