The 43-year-old woman was struck on the back of the head and knocked to the ground after she went to a woman's aid in the Countdown supermarket carpark in Northcote, Auckland, this afternoon
Police said a young man had tried to snatch the handbag of an Asian woman about 1.40pm.
The 43-year-old good samaritan, who was at the supermarket with her two young children, saw the incident and went to help.
The offender struck her on the back of the head, knocking her to the ground.
She was taken to Auckland Hospital with a fractured skull and bleeding on the brain, and is due to undergo surgery tonight, police said.
Her two children, aged 2 and 4, were not injured.
The offender, described as a young Maori or Pacific Islander, possibly 15 or 16 years old, fled empty-handed and has not been found.
He was wearing black clothing and a baseball cap.
"This woman was acting as a good samaritan in coming to the aid of someone who was being attacked," Detective Senior Sergeant Stan Brown said.
"She's sustained a serious injury in front of her own children and we're determined to identify and apprehend the offender as soon as possible.
"If you were there, please call us," Mr Brown said.
Police want to speak to anyone who was in or around the Countdown supermarket, and from the Asian woman with the handbag, who left the scene.
CCTV footage from the area is also being reviewed.
Anyone with information is asked to call North Shore Police on 09 477 5261, or Crimestoppers on 0800 555 111.
