Mr Frandi was unaware of his nephew being involved in the incident in the Waitaki Valley when contacted.
Mr MacDonald said that on Sunday, "we'd probably been going for about 45 minutes, I suppose, and then came down a hill and a girl was sort of sitting on a bike, waving for people to stop".
"Our group stopped and noticed the guy lying on the ground under a tree. We thought he was still alive, we thought he was warm, but it turned out once the ambulance got there that he was dead.
"I just watched the news again and it just showed a picture of the guy they were looking for in relation to it and that is the guy. I'm sure of it, just that haircut and the greyish hair."
Mr MacDonald said he didn't see the woman's body.
"I shot away on my bike and another of my friends was there and he waved me down to say that there was a dead woman in the trees, about 50 metres up the track."
Area commander Inspector Dave Gaskin refused to say how the man and woman died.
The pair were not thought to have been dead for long when the bodies were found.
The bodies have been flown to Christchurch for post-mortem examinations.
Mr Gaskin said police expected to confirm the names of the dead by this morning, once next of kin had been contacted.
Inquiry chief Detective Inspector Greg Williams indicated that the deaths probably occurred at the scene.
He said the police were still looking for a vehicle of interest.
Earlier, they appealed for sightings of a silver 1997 328i BMW saloon, with the registration ATT530, last seen parked at Jason Frandi's home about 11am on Saturday.
Police have set up a wide cordon around the scene.
- additional reporting Oamaru Mail