The man said he heard the young man say to the woman something like: "He's trying to take my girlfriend."
Minutes later, they saw their neighbour - the victim - walking towards the top of the driveway.
Not long after, they heard screams and someone calling for help.
"We were helping - trying to help - [the victim], but there was too much blood. It was no use." Others also rushed over to help, and the ambulance and police were phoned.
"We left it to police and the ambulance to carry out their jobs. It was bad. There was a lot of blood," he said.
Other neighbours described the victim as a "really nice guy" who lived with his partner and children.
He was treated at the scene for critical injuries before being taken to Auckland City Hospital, but later died.
At the scene yesterday, police carried out forensic tests.
Blood stains could be seen on the footpath and a small black-handled kitchen knife was visible on the grass berm behind the police cordon.
Neighbour Natasha Khan said her family heard a lot of arguing about 9pm. Her grandfather, Mansoor Ali-Khan, said the victim was a "really good guy" and a friendly neighbour. Ali-Khan heard a woman "screaming for help" soon after the fighting.
Last night police urged those involved to come forward.