The MRH-90 Taipan chopper went down near the Great Barrier Reef at 10.30pm on Friday during joint military exercises with the US.

A rescue helicopter has reportedly spotted debris near the Whitsunday Islands.

Australian Defence Minister Richard Marles told reporters: "The helicopter was engaging in a two-helicopter mission, there were four aircrew aboard. Because there was another helicopter present, a search and rescue was able to commence immediately.

"As we speak to you now, the four aircrew are yet to be found, the search and rescue continues."

He added: "The families of the four aircrew have been notified of this incident and our hopes and our thoughts are very much with the air crew and their families.

"Our hopes are very much with the efforts of the search and rescue crews as they go about their work right now."

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Australia and the US are currently conducting their biennial Talisman Sabre military exercise along the Australian coast that this year involves 13 nations and more than 30,000 personnel.

It is the second emergency involving an Australian Taipan this year, after one ditched into the sea off the New South Wales state coast in March.

That helicopter was taking part in a night-time counterterrorism training exercise when it ran into trouble. All 10 passengers and crew members were rescued.