An Australian Army helicopter has crashed into waters off Hamilton Island in Queensland during joint military exercises.
Four people were on board the MRH-90 Taipan helicopter when it went down at about 10.30pm on Friday, Defence Minister Richard Marles confirmed.
A search is under way to find the crew and their families have been notified.
Mr Marles said the missing helicopter was taking part in a mission that involved a second chopper.
"Defence exercises which are so necessary for the readiness of our defence force are serious, they carry risk," he told reporters on Saturday in Brisbane.
"As we desperately hope for better news during the course of this day we are reminded about the gravity of the act which comes with wearing our nation's uniform."
Chief of the Defence Force General Angus Campbell said Queensland authorities, members of the public and US personnel were taking part in the search.
"Our focus at the moment is finding our people and supporting their families and the rest of our team, this is indeed a terrible moment," he said.
The bulk of Talisman Sabre exercises are being held in regional Queensland.
Earlier in July, a US Army tank was caught up in a fiery crash near Rockhampton that left six people in hospital.
Earlier fatal air crash
The helicopter crash comes after two people died when two light planes collided midair at an airfield in regional Queensland.
A couple in their 60s were killed when a Jabiru J430 was taking off from one runway while a Piper Pawnee was landing on a crossing runway at the Caboolture aerodrome, north of Brisbane.
The Australian Transport Safety Bureau is investigating the crash, which happened about 10.30am on Friday.
