SYDNEY (AFP) - Australia said Friday it was buying two long-distance C-17 Globemaster planes in an A$1 billion (S$1.05 billion) procurement that will boost its military and disaster relief operations worldwide.

The Royal Australian Air Force has conducted aerial operations in Iraq against Islamic State in Iraq and Syria militants, and provided humanitarian aid in natural disaster zones in Vanuatu, Japan, New Zealand and at home.

"These aircraft make the strong arm of Australia longer than would otherwise be the case because these aircraft, particularly with aerial refuelling, can go potentially right around the world," Prime Minister Tony Abbott told reporters at the Amberley air force base in Brisbane.

"Anywhere in the world where there is an airstrip capable of handling these aircraft, we can get to within about 24 hours thanks to these planes and the refuelling capabilities that the KC-30s give us.

"So this is a very important addition to Australia's global stature."

Of the A$1 billion, A$300 million will be spent on upgrading Amberley's facilities to cater for the new Boeing aircraft as well as for existing Airbus KC-30 refuelling tankers.

Australia already has six C-17s, which have three times the capacity of the C-130 Hercules and can carry up to 77 tonnes.

"On many occasions we've flown consistently all six aircraft every day," Air Marshall Geoff Brown, head of the RAAF, said alongside Abbott.

"I think it's often said that Australia's unique geography provides us with strategic depth.

"However, that same geography necessitates a robust, flexible and responsive air mobility fleet that has strategic reach and with the announcement today I think that we do guarantee that for the nation."

Australia last year beefed up its air power with the A$12.4 billion purchase of 58 more F-35 Joint Strike Fighters to bring its total JSF force to 72. The first are due to enter service in 2020.

The JSF, manufactured by Lockheed Martin, is the US military's most expensive weapons programme ever.