She has been taken to Auckland Hospital in a serious condition.
She has a fractured skull and bleeding on the brain and is due to undergo surgery tonight.
Her family are with her and her children, who are aged 2 and 4 years old, were not injured.
The offender fled the scene empty-handed.
He is described as a young maori or pacific islander, possibly 15-16 years old.
He was wearing black clothing and a baseball cap.
Police want to speak to anyone who was in or around the Countdown supermarket at around the time the incident happened.
Detectives are tonight in the process of gathering and reviewing CCTV taken from the area.
"This woman was acting as a good Samaritan in coming to the aid of someone who was being attacked.
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" says Det Snr Sgt Stan Brown .
Police also want to hear from the Asian woman with the handbag, who left the scene prior to Police arrival.
Anyone with information is asked to call North Shore Police on 09 477 5261.
Information can also be given anonymously to the organisation Crimestoppers on 0800 555 111.
