The incident happened about 10.30pm on Monday, when emergency services were called to Marne Rd, in Papakura, South Auckland.
Police said the victim had suffered life-threatening issues.
But they did not know who the man was.
The next day, officers put out a public appeal for anyone who may be able to identify the man now in hospital fighting for his life.
The family member said her relative had suffered broken bones and serious head injuries and he had already undergone a number of operations.
He was back in surgery on Wednesday and although it is still early days, she said, he was doing well.
"He's moving his little finger a little bit at this stage."
Second time the whānau has dealt with such a tragedy
The family member said the victim, who lives in the area, had been at a friend's place nearby and had been walking home when the incident happened.
The whānau had been told the crash happened near a roundabout on Marne Rd.
It is not the first time they have dealt with a car-versus-pedestrian tragedy; after a 20-year-old family member died when he was hit by a vehicle near the Southern Motorway in Otara, South Auckland, in January.
"It just reminded us of that time," the family member said.
"We're all just doing karakia now and waiting for after his surgery."
Police have previously said that the driver of the vehicle involved is assisting police with their investigation into the circumstances of the crash.
Authorities have yet to say whether the driver will face any charges relating to the incident.
The latest update was that the investigation is ongoing.