Police believe one of the vehicles crossed the centre line.
"The couple escaped, miraculously, after their Land Rover Discovery left the road and rolled down a 30m bank."
Mr Watkins said both walked away with minor injuries.
State Highway 73 between Otira and the Waimakariri River Valley Rd was closed just after 10.30am and did not reopen until after 3pm.
Motorists heading from the West Coast to Christchurch were stopped at Otira because Arthur's Pass was packed with people returning to Christchurch after Saturday's Around Brunner cycle race.
Police are not releasing the name of the critically injured man until next-of-kin have been advised.