The weapon was found in late April by an unsuspecting fisherman on Lord Howe Island's Elizabeth Reef, with Defence personnel admitting the item could have posed "a significant risk to the general public".

The local angler was visiting the reef, about 550km off the NSW coast, with his wife. He photographed his discovery and reported it to authorities.

Navy divers aboard HMAS Adelaide carefully removed the abandoned explosive on September 25 by floating it to the surface and towing it further out to sea where it was dropped into 550-metre-deep waters.

"That depth is really safe. It's not going to ever get washed back up onto the reef," senior marine parks officer John Pritchard said.

"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 couldn't estimate its age owing to deterioration, a spokesman for Environment Minister Sussan Ley told AAP.

Bombs of that size were used as long ago as World War I, sometimes dropped from aircraft to target submarines.

The minister's spokesman said the difficult operation was complicated further by the COVID-19 pandemic and took five months to be realised, during which time the reef was closed to visitors.

Ms Ley said the fisherman and navy divers had potentially saved lives and one of Australia's most important reefs.

"The device was regarded as live by the navy and the consequences could have been quite frightening," she said in a statement on Sunday.

"Thankfully the reef's precious ecosystem is safe and most importantly so are future visitors."

Elizabeth Reef is about 160km north of world heritage-listed Lord Howe Island and spans 8.2km by 5.5km.

Along with nearby Middleton Reef, it is the southernmost coral reef platform in the world.

"There's been 125 corals identified there (and) over 300 species of fish," Mr Pritchard said.

"It's quite a unique environment and, because it's so far from anywhere, it's relatively untouched."

Australian Associated Press