The Royal Commission into National Natural Disaster Arrangements on Wednesday heard how Defence Force personnel helped during the devastating summer fires.

Lieutenant-General Greg Bilton revealed Defence was providing assistance to bushfire-affected areas as early as September 2019 under emergency Defence Assistance to the Civil Community or DACC arrangements.

This was prior to the well-publicised call-out of the Reserves on December 31. However Lieutenant-General Bilton characterised Defence's earlier assistance to parts of northern NSW and south-eastern Queensland as "modest".

"The response was essentially aviation in nature or logistics in nature," Lieutenant-General Bilton said.

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It included fire mapping, moving firefighters and goods between different fronts, and providing meals and accommodation.

Once the call-out order went out, Defence had two main roles - supporting the fire fight and providing relief, Lieutenant-General Bilton said.

"We were now working in four jurisdictions concurrently. In New South Wales, the ACT, Victoria and South Australia. So the scale of it had grown immensely.

"And then behind that, was this other task or major mission which was providing that immediate relief, and in - it involved in essence a whole range of tasks but getting into remote localities, areas where people were cut off because of fires and the routes in and out of areas had been closed off as a consequence, and being able to provide fuel, food, immediate medical care, flying in pop-up teams from human services to give access to people for emergency funding."

So-called "Operation Bushfire Assist" - while declared on December 31 - was backdated to September 6 take in the full cost of the earlier logistical support.

The operation had cost $66 million so far out of the nearly $88 million allocated. Lieutenant-General Bilton said the federal government had chosen not to recover this money from the states. The final cost of the operation would be known at the end of the financial year.

Around 8000 Defence Force Personnel assisted with the bushfires from September to March, including more than 2500 reserves. Approximately 500 soldiers from abroad also helped, from countries including New Zealand, Papua New Guinea, Japan and Fiji.

They moved more than eight million litres of water; 73,000 litres of fuel; 13,000 tonnes of fodder; and 1200 tonnes of air cargo. They cleared 4850 kilometres of road, repaired 1280 kilometres of fencing and cleared 240 kilometres of fire breaks. They also prepared 77,000 civilian meals and evacuated 527 people.