The bomb was found by a fisherman on Elizabeth Reef near Lord Howe Island, about 340 miles off New South Wales, and he photographed his discovery and reported it to authorities.
Divers aboard the HMAS Adelaide removed the abandoned explosive by floating it to the surface and towing it farther out to sea where it was dropped into 550 metre deep waters.
Although there was no date given for the bomb removal, the Australian Department of Defence photos from the mission were dated September 25.
Senior Marine Parks Officer John Pritchard said: “That depth is really safe. It’s not going to ever get washed back up onto the reef.
“There’s no deep-sea fishing or trawling allowed out there. It’s a recreational fishing zone only. The chances of that UXO (unexploded ordinance) ever coming back to the surface is negligible.”
The origin of the bomb is not known and divers could not estimate its age because of its deterioration, a spokesman for Environment Minister Sussan Ley said.
Bombs of that size were used as long ago as the First World War, sometimes dropped from aircraft to target submarines.
There were also bombings off the Australian east coast during the Second World War.
The device was regarded as live by the navy and the consequences could have been quite frightening
Ms Ley said the fisherman and navy divers had potentially saved lives and one of Australia’s most important reefs.
She said: “The device was regarded as live by the navy and the consequences could have been quite frightening.
“Thankfully the reef’s precious ecosystem is safe and most importantly so are future visitors.”
Elizabeth Reef is about 100 miles north of World Heritage-listed Lord Howe Island.
Along with nearby Middleton Reef, Mr Pritchard said it is the southernmost coral reef platform in the world.
He said: “There’s been 125 corals identified there (and) over 300 species of fish.”
“It’s quite a unique environment and, because it’s so far from anywhere, it’s relatively untouched.”
