The federal Environment Department says it will investigate a fire started by Energy Resources Australia (ERA) that spread into Kakadu National Park, threatening important cultural sites.

The fire started at ERA's Ranger uranium mine a week ago and spread into the World Heritage-listed park, threatening several culturally sensitive Indigenous sites.

In a statement to the ABC, a spokesman for Federal Environment Minister Greg Hunt described the fire as a "very serious matter".

"Minister Hunt has asked the Department and Parks Australia to conduct a full and thorough investigation into the cause of the fire," the statement said.

"No permission was sought and no approval was received for the lighting of the fire by ERA.

"We will not hesitate to seek reimbursement for the costs of firefighting if negligence or wrongdoing are in any way shown.

"Additionally, a breach of the Environment Protection Biodiversity Conservation (EPBC) Act can result in fines of up to $8.5 million."

The ABC understands the NT Department of Mines and Energy will also be investigating the fire.

We don't need approval to burn: ERA

Ancient Aboriginal rock art at Nourlangie in the Kakadu National Park, Northern Territory, some of which dates back 50,000 years, date unknown. ( Martin Kwakwa, file photo: AAP )

ERA has issued a statement saying it did not need approval to burn-off in its Ranger Project Area.

It said it had "followed its normal protocol to notify stakeholders prior to undertaking weed management activities" and was "not required to seek approval or obtain a permit for such activities on the Ranger Project Area".

The company, which is majority owned by mining giant Rio Tinto, said it would "cooperate fully with any investigation" by the Department of Mines and Energy.

"On the day of the weed management activity, there was no fire ban in place for the area or the region in which the Ranger Mine is located," the statement said.

On Wednesday ERA issued a statement saying it would cover the cost of aerial water bombing and it was "working with Parks [Australia] to determine the cost".

Aboriginal groups angry over fire

Justin O'Brien from the Gunjeihmi Corporation, which represents the area's traditional owners, said ERA needed "to be taught about the sensitive environment" they operate in.

"There's an argument to say they should be prosecuted for what they've done, this is the second year in a row that they've done this, It's almost a replica of last year," he said.

"They are not learning so they need to be taught about the sensitive environment which they're operating in."

The Northern Land Council (NLC) said it was not confident a federal investigation would find anyone accountable for the fire.

Joe Morrison, CEO of the NLC, said he wanted to see traditional fire management practices reinstated.

"There's been lots of fires and lots of investigations in relation to Kakadu and surrounds for a long time, we wouldn't want to hold our breath," Mr Morrison said.

Mr Morrison said he wanted to see Aboriginal people "take control of that agenda and reinstate their traditional fire management practices".