Japanese World War II bomb discovered in Darwin Harbour near CBD

Excavators sifting through soil at a construction site on the edge of Darwin's CBD turned up an unnerving find on Monday: an unexploded 60-kilogram World War II-era Japanese bomb.

The bomb squad was swiftly called in to assess the unexploded ordnance, before clearing the area, burying the bomb in a hole, covering it with sandbags and blowing it up on site.

The Australian Defence Force (ADF) confirmed the "high-explosive" bomb was found by local contractors, who had dragged the bomb into an excavator bucket before promptly placing it back on the ground and calling authorities.

Kyle Johnson, an explosive ordnance disposal technician with the ADF, was contacted by police early on Monday afternoon, he said, about a possible grenade being discovered at the Darwin Waterfront.

"We get a lot of grenades come through … [but] a couple of minutes later the police called confirmed it wasn't [a grenade] and was about 45cm long, so that triggered us," he said.

"From there we loaded up the vehicle … and we proceeded down to the Waterfront to get eyes on."

Mr Johnson was the one to approach and assess the bomb after authorities had shut down the wharf.

"I got eyes on it and confirmed straight away that the fuse was still intact and still in there," he said.

"The fuse is the dangerous part of the majority of explosives.

"Even though [the bomb] had been initially dropped 80 or so years ago … there's still a big risk of it functioning."

Mr Johnson said that while the explosive power of unexploded bombs may reduce over time, their sensitivity can increase.

"If anything [the bomb] was more dangerous now than what it was in World War II," he said.

"The fuse mechanisms on some of these things is effectively a firing pin held back by a spring … a last little bit of movement could release the firing pin and cause the bomb to function.

"No [unexploded ordnance] is safe. Don't handle it. Call in the police, and go from there."

Ensuring there would be no unexpected or unsupervised explosion of the bomb, the ADF forced its detonation.

"We had to make sure no aircraft flew over … or no boats were around," Mr Johnson said.

"We got the contractor to dig a 1.8-metre hole … we basically then moved it, put it in the hole, covered it with two pallets worth of sandbags, hooked up our own explosives to it, and blew it up."

A statement from the ADF said the bomb was "determined live and unsafe to transport ... [and] was disposed of on-site using routine measures".

Unexploded bombs not such a rare find

On February 19, 1942, Japanese aircraft launched the largest attack ever mounted on Australian soil, known as the Bombing of Darwin.

Japanese aircraft dropped an estimated 680 bombs over the city, killing up to 250 people. The attack would go on to last 20 months.

The legacy of the attack runs deep in the city's memory, and also includes the frequent discovery of potentially viable bombs.

"It's reasonably common that happens. There's actually live ordnance all over the Top End, some from enemy attacks, and much of it just from the Allies training as well," Jared Archibald, a historian and curator at the Museum and Art Gallery of the Northern Territory (MAGNT), said.

"Especially around places like the Marrakai Track and the bottom of Litchfield … in the back country hunting areas where lots of training was done.

"[It's often] found over time as bushfires go through and people stumble across it."

Mr Johnson said across Australia this year technicians had dealt with 420 jobs relating to unexploded ordinance. Ten of these have been in the Northern Territory.

"This year alone we've done a grenade at a caravan park in Katherine, we did a grenade just behind Don Dale prison, and then last year [there were] quite a few jobs in Litchfield.

"So, it's quite busy."