A large out-of-control scrub fire has shut a main road between Canterbury and the West Coast.

The fire broke out on conservation land at around 4:30am on Sunday and quickly grew into a raging monster. The blaze spread to both sides of State Highway 73 just west out of Springfield - a critical link between Canterbury and the West Coast.

It's eaten through thick tussock, broom and even snow, covering over 150 hectares by midday.

Crews from around the Canterbury have been tackling the blaze, but because of the steep terrain six helicopters with monsoon buckets have had to be used.

Fire and Emergency North Canterbury area commander Bruce Janes has been surveying the damage from the air. He warns that these fires can turn into an "ecological nightmare".

"The fire's still out of control and not contained," he told Newshub.

"It's all aircraft attack at this stage, with ground crews just starting to do so."

SH73 is the main road to the West Coast, and motorists still hoping to make it there on Sunday will have to head north through Lewis Pass - adding about five hours to their journey.

Fire and Emergency expect to fight the fire into Monday.