Ten others were hospitalised in Townsville, home to a large army base in the tropical northeast about 700 miles (1,100 kilometres) from Brisbane.
Three were critically injured.
A spokesman for the Australian Defence Force, Brigadier Adrian D'Hage, said the two helicopters had been flying about 50 metres (165 feet) above the ground in darkness when they collided at speeds of 70mph (113kph) to 100mph (160kph).
Both crashed and burned, scattering debris over the hilly terrain, Brig D'Hage said.
The toll took several hours to confirm as some of the victims had to be located in scrub land. An army spokeswoman later said six died at the crash scene.
Another 12 died in transit or after arrival at Townsville Hospital.
The total number of crew and soldiers on board both craft was 28. All belonged to the elite Special Air Service commando regiment.
Brig D'Hage described the dead as being some of the army's "finest men". However, he refused to speculate on the cause but said the crew had been using night-vision goggles. The accident had happened as they came into land.
He said the two helicopters had been taking part in a twice-yearly exercise that simulates a counterterrorist attack.
The US-built Sikorsky Blackhawk helicopters are used extensively by the Australian army.