A man disguised as a woman and armed with a "homemade machine gun" has robbed a service station in Melbourne's north-west.

The cross-dressing thief entered a service station on Taylors Road in Taylors Hill about 7:30pm last Monday.

Police said he confronted a staff member with "what appeared to be a homemade machine gun" and demanded money.

The employee handed over cash before the thief fled the store.

There were customers in the store at the time but no-one was injured.

Police released two images of the man captured on security cameras.

Anyone with information is urged to contact Crime Stoppers.