Alex Bhathal's nine-year-old son was the first person to notice the graffiti on his mother's Thornbury campaign office during his morning bus ride to school.

Victorian police are investigating who scrawled "Liberal bitch", "sell out" and "liar" in black marker across the glass windows of Bhathal's office.

The Greens candidate says the sexist words have left her feeling unsafe running her campaign.

"I've got limited protections as a volunteer, it's not like I can go off to my boss and say 'what can we do about this'? It's not good.