"There was 10 minutes of intense hail which formed an inch of ice making driving extremely difficult."
Mr Owen said they closed the road and managed to get most people off the hill by 9.30pm, although a truck driver had to leave his vehicle behind.
Mr Owen said crews worked hard to reopen the road about 11.30pm and stayed to clear the road as snow showers fell.
MetService key account manager Rob Hamilton said temperatures recorded at the summit showed it was 1.2C at 6pm and then dropped to -0.6C at 7.30pm.
He said a little rain fell from 3pm and into the evening.
"When it's cold like that, obviously any precipitation is going to fall as hail and snow."
Mr Owen said normally, if ice was predicted, road crews would lay grit and calcium magnesium acetate, a de-icing agent.
While it continued to snow into early yesterday morning, the road remained open.
Snow closed other key routes in the Central North Island and Cook Strait ferries were cancelled yesterday morning.
The Desert Rd was closed from Taihape to Rangipo, as was the main detour from Ohakune to Waiouru.
Fire Service communications centres in the North Island said there had been reports of trees and power lines down.
Power outages also hit Alfredton, Makuri, and Gladstone just before midnight on Thursday.
