THE US military says it was trying to avoid errant boaties when it dropped four unarmed bombs on the Great Barrier Reef.

Two US Marine Corp jets were forced to drop four unarmed 226kg bombs on the world-heritage listed Great Barrier Reef last week after an emergency situation arose in a combined US and Australian Defence training exercise.

The US military says it had to dump the ordinance in an unplanned location after "civilian vessels'' were detected inside a pre-approved drop zone.

Commander William Marks, of the US Seventh Fleet, said the crews had no other option.

"The approved area where they could do some of this live training with these 500-pound bombs, it was not safe to drop the bombs," he told ABC radio.

"There were civilian boats right below them."

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Defence personnel yesterday said they would retrieve the bombs if ordered to do so.

The Great Barrier Reef Marine Park Authority's chairman Russell Reichelt said while it was not an ideal situation the consequences of the dumping were minimal.

"The impacts on the environment are negligible and since there was four weapons dropped, two were filled with concrete and two with explosives, they weren't armed and they were essentially inert so the environment affects where they land on the flat sea bottom,'' he said.

"It was well away from any sensitive habitat.

"It was a habitat protection zone which is an area of intermediate protection in the Great Barrier Reef, it wasn't an area in the marine national park zone and it wasn't a trawling zone.''

The incident occurred northeast of Townshend Island, off the Queensland coast northeast of Rockhampton on Tuesday and GBRFP was notified the following day.

It was part of a training activity prior to Exercise Talisman Saber - a combined US and Australian Defence training exercise.

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The aircraft was running low on fuel and resulted in personnel having to dump the four unarmed pieces of ordnance in an area about 80km from the shore.

Mr Reichelt said divers could retrieve the bombs, which were three-metres long and 30 centimetres diameter, that were resting at 50 metres deep.

He said no penalties would be inflicted as it was an emergency situation.

Vice-Admiral Scott Swift of the US 7th Fleet told reporters aboard the USS Blue Ridge dropping the bombs on the Barrier Reef had been the safest option open to the pilots.

He said an investigation was continuing but if required the US Navy could return to try to collect the bombs.

- additional reporting by AAP

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