Speed was not thought to be involved. Police were waiting for the results of a driver blood-alcohol test.
The investigation was continuing, Bunn said.
The man's parents were on their way from Europe to be with him at Auckland Hospital.
He was believed to be on a working holiday in New Zealand.
A St John spokesperson said three ambulances and a helicopter responded to the incident.
One patient was transported in a critical condition to Auckland City Hospital by helicopter.
The highway was closed for several hours with traffic diverted via Oromahoe Rd and Whangae Rd.