Australian Defence Minister Richard Marles on Monday declared four army aircrew members, who were aboard a helicopter that crashed into the sea on Friday, as dead.

Efforts to rescue Captain Danniel Lyon, Lieutenant Maxwell Nugent, Warrant Officer Class Two Joseph Laycock and Corporal Alexander Naggs have now turned to an effort to recover their bodies, reports CNN.

Richard Marles was quoted as saying by CNN: "The loss of these four men is as significant and meaningful as the loss of anyone who has worn our nations uniform. If it is, as we imagine it to be, they died on Friday night making a difference."

Significant wreckage of the MRH-90 helicopter has now been located, Marles said, indicating the men likely died in the “catastrophic” incident late Friday.

Marles said the “determined recovery effort involving hundreds of defense force personnel” is underway, saying there will be a “full investigation” to understand exactly what happened.

Prime Minister reacts:

Prime Minister Anthony Albanese mourned the death of the crew members and said: "We offer our sympathies and condolences to the families, friends and colleagues of Captain Danniel Lyon, Lieutenant Maxwell Nugent, Warrant Officer Class Two Joseph Laycock and Corporal Alexander Naggs."