A woman was ambushed by man before he hit her in the head with a pickaxe in a terrifying car jacking at a petrol station in Melbourne's south-east.

The woman, believed to be aged in her 30s, was leaving the Coles Express petrol station on Springvale and Governor roads in Braeside when she was confronted by a man wielding a pickaxe at about 3.30am on May 17.

It is believed the man, who was wearing a dark hooded jumper at the time of the attack, had been hiding in a garden area at the service station before he attacked the woman and demanded her car keys.

When she refused to hand them over, a struggle ensued before the man hit the victim in the head with the pickaxe and managed to grab her keys.