An elderly woman was put in a choker-hold by a man who jumped into the back of her car, demanded her purse and stole her $10,000 pearl necklace.
Police say this type of aggravated robbery is unusual but are warning people to be wary.
A police spokeswoman said the attack was carried out in the main carpark at the Pokeno markets on Sunday around 11.30am.
The two Mt Roskill women, the driver in her 30s and her passenger in her late 70s, pulled into a parking space and had just turned the car off when the attacker jumped into the back.
The attacker, described by one witness as a man or possibly an ugly woman, grabbed the older woman tightly around her throat and shouted at the pair to hand over their handbags.
The passenger had her feet on her handbag in the footwell and the attacker screamed at her to hand it over, squeezing her throat tighter in the process, the police spokeswoman said.
The driver told police she felt something sharp pressing into her left side, meaning the attacker possibly had a weapon.
Police say the attacker took a black Guess handbag which contained cash, as well as the older woman's Mikimoto pearl necklace, worth as much as $10,000, as well as a gold cross with sapphires.
He fled, disappearing into the crowd.
The police spokeswoman said the elderly woman suffered bruising to her face, neck and hand, and both received medical treatment.
Detective Inspector Gary Hill said such an attack was unusual.
"What we've seen before and what we regard as a common method is where offenders distract shoppers and then take their property."
Hill said people should be aware of their surroundings and lock their cars to stop people getting in.
Neither of the women could provide a detailed description of the attacker, though he had strong body odour, is thought to have been Maori and was wearing a dark top.
A witness who saw the man flee said the victim was a male "or ugly looking female", wearing blue tracksuit pants.
Police want to hear from anyone with information relating to this incident or any similar incidents or repeat offenders.
They are asked to contact Counties Manukau police on 09 295 0200 or Crimestoppers on 0800 555 111.