Hume Crime Investigation Unit detectives are investigating after a heavily pregnant woman was robbed while working at a Sunbury service station on Wednesday.

A man entered the Horne Street servo when the female attendant was getting ready to close the store about 9.55pm.

He has demanded cash which she has handed over.

Unhappy with the amount provided, the man has become aggressive and told the woman he had a gun and threatened her.

The woman has told him that she had provided him with all the cash and the man has left.

The 8-month-pregnant woman was not injured during the incident.

Police have released below images of a man they believe can assist with their enquiries.