DOC believes a large bush fire in the back of Canterbury's Big Ben Range at the weekend was deliberately lit.

Ground crews couldn't reach the fire at 1600m above sea level on Saturday, so the Department of Conservation used helicopters with monsoon buckets to fight it until the evening.

Initial suggestions were that a light aircraft may have crashed there, but that turned out to be unfounded.

North Canterbury Operations Manager Kingsley Timpson said there was no lightning at the time so they want to speak with trampers and hunters.

"We believe that people were present at the time, or seen in the area shortly before the fire, but it hasn't been confirmed yet and an investigation is under way."

It could take 20 years for bush burnt to grow back.

Timpson said it's a huge setback for the native vegetation, birds, geckos and insects in the area.

"It could be in the order of 20 years before we see ground cover back to the same height."