He said the man was the sole occupant in the vehicle, and it appeared he may have had an "event", as he was allegedly driving erratically and crossed the centre line.
Dr Raumati said the elderly man was transported by rescue helicopter to Auckland City Hospital in a critical condition.
He said other patients were taken by St John Ambulance to hospital, in moderate conditions.
NZTA says motorists should expect delays and follow emergency services' directions.
Update: SH1 is CLOSED north of Puhoi. A detour is in place, which will add at least 60mins to your journey. ^LC pic.twitter.com/RXfTprgwht
— Waka Kotahi NZTA Auckland & Northland (@WakaKotahiAkNth) August 20, 2015
On-ramps to SH1 heading north are closed at Silverdale, Grand Drive and Hibiscus Coast Highway.
A detour has been put in place which will add at least one hour to northbound travel.
Paul Nicholas was heading south when the crash occurred.
"Twenty seconds earlier and that could have been me," he said. "The cars were pretty mashed."
The cars collided on one of the last bends before the Puhoi exit, Mr Nicholas said.
One car caught fire, but the blaze was soon put out, he said.