Police are hunting a bag snatcher who pushed a pregnant woman to the ground and smashed her husband in the head with a hammer outside a McDonald's restaurant.

The couple were walking to their car outside the Royal Oak McDonald's in Mt Albert Rd at 2.20pm on Friday when a man ran up.

He grabbed the 18-weeks-pregnant woman's bag and the pair struggled before she fell to the ground. Her husband chased the thief but was then attacked himself.

Senior Sergeant Hirone Waretine said the man reached into the thief's car to try to get the bag back and was dragged for some distance.

"The male and a female in the car were trying to push him out, and then the husband was smacked in the head with what he thought was a hammer," Mr Waretine said.

The thief and his passenger took off in a stolen Nissan Cefiro, registration number UW2682.

The couple were taken to hospital where the man was treated for head injuries. His wife and their unborn child were not injured.

In April, police revealed that a group of young male bag-snatchers were targeting women returning to their cars from shopping at busy Auckland supermarkets.

A source told the Herald that the notorious Killer Beez youth gang was at the top of the suspect list.

Mr Waretine said Friday's incident was likely to be connected to the earlier bag snatches, with the offender being part of the group responsible or closely associated.

In 2008, a group of men stole businesswoman Joanne Wang's handbag at Manukau City shopping mall. They took off in a stolen vehicle, hitting Ms Wang, who died two days later.

Police urged women to be vigilant and lock their handbags in their cars before unloading groceries.