Great Barrier Reef.

Four unarmed bombs have been dropped on the Great Barrier Reef. Photo: Supplied.

Authorities are yet to decide whether they will recover four unarmed bombs dropped onto the Great Barrier Reef during military exercises.

Two of the bombs did not contain explosives and the other two were without their fusing mechanisms.

US warplanes were forced to drop the bombs after they ran dangerously low on fuel during a problem-plagued training exercise in central Queensland last week.

The planes were unable to drop the ordnance where they had intended because civilian vessels had wandered into the pre-approved drop zone.

Instead they were let go in about 50 to 60 metres of water inside the boundaries of the Great Barrier Reef marine park.

United States 7th Fleet spokesman Lieutenant David Levy says the Australian and US governments are still determining whether recovery is possible.

"If the (Great Barrier Reef Marine) Park Authority and government agencies of Australia determine they want them recovered, then we will coordinate with them on that recovery process," he said in a statement issued late on Tuesday.

In answer to a query about whether one of the two bombs containing explosives could go off, Lt Levy said: "Talking with explosive ordnance experts, it is virtually impossible for it to explode."

He was unable to say why civilian boats were in the drop zone, given it was standard procedure to warn the public about exclusion sites.

"Safety to life is our most important priority, therefore range authorities decided not to drop the bombs."

An investigation into the incident is underway, including whether any negligence was involved.

The bombs were dropped during a training activity prior to Exercise Talisman Sabre, a joint war games involving US and Australian troops.

AAP