Four people are feared dead after an Australian Army helicopter crashed into the ocean off north Queensland - the second emergency involving the trouble-plagued MRH90 Taipan fleet in less than six months.

It is not clear yet what caused the helicopter to ditch into waters near Hamilton Island about 10.30pm on Friday (local time) during a two-helicopter training mission.

But it came just months after another incident involving a Taipan helicopter off the New South Wales south coast.

In March, 10 ADF personnel - including special forces soldiers - had to be rescued after the MRH90 ditched into the sea at Jervis Bay during counterterrorism exercises.

The incident left two people with minor injuries and prompted an immediate grounding of the fleet while investigations were carried out.

The Taipan helicopters were back flying again in April, with "risk mitigations" in place.

Later that month, the ABC revealed that military figures claimed a software upgrade had not been installed on all helicopters in the fleet despite warnings it could be needed to prevent possible engine failures.

Bought for use by both the army and the navy, Taipans were first introduced to service in Australia in 2007, however concerns saw some or all of the fleet grounded in 2019 and 2021.

In 2021, then-defence minister Peter Dutton said the performance of the MRH90 Taipan had been an "ongoing and well-documented concern for Defence" and there had been a "significant effort at great expense to try to remediate those issues".

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At the time, the Morrison government revealed Australia would ditch the fleet a decade earlier than expected, seeking to replace them with American Blackhawks and Seahawks.

"In this case, sadly the Taipans didn't do what we needed them to do so we'll get the things we need to do it," then-prime minister Scott Morrison said.

The army plans to withdraw the Taipans from service in 2024.

"With a large cabin, rear ramp, weather radar and forward-looking infra-red, the MRH90 can transport greater loads and carry more troops in adverse conditions," the Defence website states.

"The MRH90 Taipan capability will continue to contribute to Defence requirements until it is withdrawn from service in 2024," it adds.

Speaking after Friday's incident, Defence Minister Richard Marles said defence exercises, while necessary, "carry risk" and that the crash was a reminder of "the gravity of the act which comes with wearing our nation's uniform".

In June 1996, two helicopters collided mid-air during a Black Hawk training exercise near Townsville, killing 18 men.