A man in a Santa Claus suit has robbed a post office in Melbourne's north, escaping with cash in a red sack.

The man entered the post office in Oak Park about 11:30am on Saturday and demanded money from a female staff member, police said.

The woman gave the man cash and he put it in a red Santa sack.

He left the shop and drove away in a cream-coloured Jeep with false number plates.

Police said the man did not appear to have a weapon and no-one was hurt.