An aerial view of the damage during the first few days of the Onamalutu forest fire near State Highway 6.

Nelson Forests could face a multimillion-dollar insurance bill after a fire that started on its property destroyed vast tracts of farm and forest land in Marlborough.

The amount is still climbing as surrounding property owners in the Onamalutu area count the damage and firefighters add up the mounting cost of their largest operation in more than a decade.

The perimeter of the rural Marlborough fire that started last week spans close to 600 hectares.

It was discovered over the weekend that the blaze, which is largely under control, began on property owned by Nelson Forests.

Any damage caused by a fire that starts on commercial land is potentially liable to be paid by that company. This included the third-party loss and the cost of fighting the fire, estimated to be at least $750,000. "Expect it to go up," Marlborough Kaikoura principal rural fire officer Richard McNamara said. "We are still accumulating the cost of the fire line and water tankers - that all has to be accounted for."

The blaze was the biggest fire in the region since the Boxing Day fires, which ran across the Wither Hills in 2000. The total bill including the damage was hard to comprehend but would be in the multimillions of dollars, he said.

"There are a number of private forestry operations within the fire perimeter," McNamara said. "There is Nelson Forests and large, private lifestyle blocks. It's a complex mix of land."

Nelson Forests managing director Lees Seymour said the company's liability would depend on how the fire started but he was confident their insurance would cover all the damage.

"We are all disappointed. No one likes to see occurring on anybody's properties especially when people's lives are at risk. We are just thankful nobody was injured and there was no significant loss of property."

He said that though it was "quite a visual event" the damage sustained on the company's land was not likely to be that great and much could be salvaged.

Marlborough Mayor Alistair Sowman told Radio New Zealand that better cleaning up of forestry wastage sites was needed to reduce the threat of heat-induced bushfires.

The debris from felled trees, known in forestry as skid sites, can ignite in extremely hot weather. "They are the major causes of our fires, we need to change the practise quickly."

Seymour said the preliminary indication was that this fire did not start in this way. However, the investigation was still underway.

When the cause was discovered the case would become a "conversation between insurance companies" as to who paid what.

About 100 firefighters are still deployed.

Incident controller John Foley, of the Marlborough-Kaikoura Rural Fire Authority, estimated that there were three more days of solid work to be done on the ground but it was hoped that responsibility for dealing with the aftermath of the fire could be handed back to Nelson Forests later this week.

Waimea Rural Fire District principal rural fire officer Ian Reade said eight fire crews from the Nelson region were fighting the fire, which meant resources were stretched. They had cancelled 102 permits in the Nelson region last week. "We went into lock-down mode around here," he said. "We are wary that fire could still make a run."