Helicopters have been called in to fight a large bush fire in Canterbury.

The blaze has a 300m front, and is burning on the Big Ben range in Rakaia Gorge.

Southern fire communications shift manager Andrew Norris said the blaze is high up on the range and was inaccessible via land.

"The Department of Conservation has sent two helicopters up there to fight it and we'll be assisting with two crews from Hororata.

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"It's well up near the snow line so there's no access to it."

The Fire Service was notified of the fire by a caller who said they heard a loud explosion.