Mr Donker, of Whangarei, said he thought the BMW sedan had a blowout on its journey from Kaikohe to Whangarei. He was in the front, his boss had been driving and there was another male in the back.
"I heard a bang, we went sideways and then we turned over," Mr Donker said.
The officer in charge at the scene, Sergeant Graeme Pugh, said early indications were that the southbound BMW had flipped after its driver lost control while overtaking two other cars.
He was waiting for police Serious Crash Unit officers to arrive from Whangarei to assess the accident scene.
The Hikurangi Volunteer Fire Brigade attended the crash, as did the Towai Rural Fire Party and a Whangarei Fire Brigade rescue tender.
Mr Buckton said a St John Ambulance crew, which was first to arrive, had the injured driver "pretty well sorted" when the Hikurangi appliance got there.
Firefighters stabilised the overturned car before cutting the driver's side so the injured man could be easily removed.
They also stopped traffic to ensure the safety of emergency crews and provide space on the road for the Northland Electricity rescue helicopter to land.
Traffic backed up for more than a kilometre either side of the scene for about an hour.