The Royal Australian Air Force will soon receive two more heavy-lift C-17 transport planes as part of a $1 billion purchase.

The announcement was made by Prime Minister Tony Abbott at the RAAF Amberley Base in Queensland on Friday.

Australia already has six of the powerful Boeing C-17 Globemasters.

The planes have been involved in diverse operations, from humanitarian drops in the Pacific, to the repatriation of bodies following the MH17 tragedy.

The Government said the two additional C-17s will provide vital heavy airlift support to a range of regional and global coalition operations and greatly increase Australia's capacity to provide rapid and effective disaster rescue, relief and humanitarian aid.

Mr Abbott announced the purchase during a visit to RAAF Base Amberley in Queensland, where the C-17s are based.

The billion dollar investment includes $300 million to upgrade the facilities at Amberley.

"These aircraft make the strong arm of Australia longer than would otherwise be the case," Mr Abbott said.

"These aircraft, particularly with aerial refuelling, can go potentially right around the world, 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."

RAAF Chief Air Marshal Geoff Brown said the additional aircraft will help the Air Force continue its humanitarian work around the globe.

"The 7th and 8th aircraft that have been announced today and the initial, additional facilities and people will allow 36 Squadron to continue and sustain that rate of effort well into the future," he said.