The arrival from the United States of the first two of four air tankers promised by Prime Minister Scott Morrison to help with national firefighting efforts has been delayed by tornadoes in Alabama and an erupting volcano in the Philippines.

Key points: $20 million has been provided to lease two DC-10 and two MD-87 air tankers

$20 million has been provided to lease two DC-10 and two MD-87 air tankers The first two aircraft are expected later this week with the other two next week

The first two aircraft are expected later this week with the other two next week The Japanese Government has offered two military transport aircraft to help with bushfire relief operations

Richard Alder, general manager of the National Aerial Firefighting Centre (NAFC), which manages the procurement and coordination of firefighting aircraft for the states and territories, told the ABC the first two — a DC-10 and an MD-87 — were now expected to arrive later this week.

Mr Alder said the DC-10 had been undergoing maintenance in Alabama when the area was hit by an extreme weather event.

"Somewhat ironically, the DC-10 which was due to depart from the United States around now has been delayed by tornadoes in the Gulf Coast area of the US and the first MD-87 aircraft has been delayed by a volcanic ash cloud along the route," Mr Adler said.

The second two aircraft — another DC-10 and another MD-87 — should touch down in Australia next week.

Additionally, a fleet of water scooping planes, lauded as the champions of Canada's firefighting fleet, are iced in and cannot make it to Australia to help fight fires.

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The delays come as the Japanese Government prepares to send two C-130 Hercules military transport aircraft from its Self Defence Force to Australia to help with bushfire relief operations.

Already Australia has received military assistance from neighbours such as Singapore, Papua New Guinea, Fiji and New Zealand.

The Royal Australian Air Force flew urban search and rescue crews to Japan following the 2011 earthquake and tsunami.

The delivery of one of the planes has been delayed by ash from the eruption of the Taal volcano in the Philippines. ( AP: Bullit Marquez )

Aircraft to touch down imminently

Mr Morrison announced on January 4 that the Federal Government would provide $20 million for the NAFC to lease four aircraft for the fire season with the first two to be available by January 11 and the others by January 18.

Mr Alder said the procurement of the aircraft was already taking longer than initially suggested.

"Once we had engaged and confirmed the funding and engaged the aircraft, we were hoping that the first tranche would be here the middle of this week," he said.

"It looks like being delayed really by … one to two days from that, so long as the weather does allow that departure to happen in Alabama."

Premium for short notice

Mr Adler also conceded that leasing the aircraft at short notice meant they were more expensive than they would have been otherwise.

"That's part of the job of juggling risk versus the financial costs of the arrangement, but it is certainly true that there is some premium for short notice," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 24 seconds 1 m 24 s An aircraft bombs a bushfire in the Sydney suburb of South Turramurra in November last year.

However, he could not quantify how much extra it was costing taxpayers.

"I couldn't really compare them because the other planes that we lease are different sort of aircraft on different arrangements and for different periods of time," he said.

"It's very difficult to compare, but I can say that the prices that we do pay for these aircraft are still very reasonable in a competitive market."

Aircraft procurement a 'complex issue'

In response to a request for comment, a Federal Government spokesperson said that the National Aerial Firefighting Centre — a business unit of the Australasian Fire and Emergency Service Authorities Council — managed the procurement of water bombing planes separately from the Federal Government.

"The matter of transporting aircraft and support logistics is a complex issue and it is rightly left for operational agencies to manage," the spokesperson said in an emailed statement.

"The Australasian Fire and Emergency Service Authorities Council requested one water bombing plane at 8pm on 3 January and the Federal Government responded the next morning with $20 million in funding for four water bombing assets."

A boost to our firefighting fleet

The addition of the four new aircraft to Australia's 150-strong firefighting air fleet will bring the total number of large or very large air tankers available this fire season to 11.

Mr Alder said the large and very large tankers were capable of dumping a huge amount of fire retardant, foam or gel on a fire in a short space of time.

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The DC-10s are capable of dropping about 30,000 litres along a fire control line of about 1 kilometre while the MD-87s can drop about 11,000 litres over about 400 metres.

"Because of their speed, they can go back to a base, reload and quite quickly be on scene again," Mr Alder said.

"So they can build quite a long control line in a relatively short space of time."

A DC-10 water-bomber was leased to assist with bushfires in New South Wales in 2016. ( Supplied: Tasmania Fire Operation Centre )

The plan is to initially base the aircraft at Canberra, Sydney, Adelaide and Perth and move them around according to need.

"This will be the first time we've had the capacity to position large air tankers in South Australia and West Australia on an ongoing basis," he said.

"Previously, we've only been able to sort of shift them there on a short-term basis according to the risk at the time, so that will provide a significant increase in capability.

"One of the greatest benefits from using aircraft is to use them in rapid initial attack and to shorten the attack time. So spreading them around the country certainly enhances the ability to do that."

Mr Alder said the aircraft were initially being leased for 50 days with the option of extending until the end of the fire season, which could be as late as May.

"The contracts are flexible so that we can make sure we've got the right number of aircraft in the country according to the conditions at the time," he said.