A 25-year-old who tried to stop two people from fighting at a party in South Auckland was stabbed twice in the back.

The man was at his cousin's house on Costar Pl in Manurewa about 11pm on Tuesday where a number of people were drinking, a police spokeswoman said.

After hearing a "lot of people yelling" on the balcony the man placed his wallet and phone on a barbecue and went to investigate.

The spokeswoman said he then tried to stop two men fighting but while getting in between them he was stabbed twice in the back, on his left shoulder, by the girlfriend of one of the men.

The knife used was described as being 15cm long and snapped in half during the attack.

The spokeswoman said the victim then sat down on a couch to recover, but was again assaulted by the woman.

"The female has punched him three or four times in the face."

She said the woman and her boyfriend then left the property with the victim's phone and wallet.

The man returned the next day, along with his girlfriend and her father, and recovered his stolen property.

After seeing a doctor, he was sent to Middlemore Hospital and underwent surgery for his stab wounds.

Police know the identity of the man's attacker but are yet to speak with her.

- © Fairfax NZ News