A specialist dive team has begun a complex operation to remove a suspected unexploded bomb that could have sat on the sea floor of Darwin Harbour for up to 80 years.
The Royal Australian Navy's Clearance Diving Team One was deployed to Darwin on Friday to retrieve the item that was suspected to be an old mortar shell, found during a survey of the planned site of the Darwin Ship Lift project this week.
A defence spokesman said on Saturday a team of five divers were using sonar equipment to search for the unexploded ordnance after failing to locate it on Friday.
Since being established in 1966, the team has been involved in operations in Vietnam, Kuwait, East Timor and across the Middle East, and was deployed to Darwin after the suspected ordnance.
A Department of Planning, Infrastructure and Logistics DIPL spokeswoman told the ABC earlier this week that the object appeared to be a mortar shell, though it has not yet been confirmed.
She said the object, which measured between 60 and 80 centimetres long, had been found on the ocean floor.
Darwin was one of a number of locations in Northern Australia that were bombed by Japanese forces during World War II, as part of a strategy to prevent the Allies from having an air base in the country.
The bombing of Darwin on February 19, 1942 was the largest and deadliest single attack ever mounted by a foreign force on Australian soil, with the Northern Territory capital being the main target of those raids.
Military historian Tom Lewis said a number of bombs had been left behind in the 208 raids in Northern Australia, from mortar shells fired by the army to larger bombs dropped by planes.
Sometimes the bombs don't go off when the bombs are released. He said that's fairly normal for World War II or indeed today you get a certain percentage that just don't detonate, for one reason or the other.
You've got a container full of explosive with a detonating system, and it's possible that it's going to go off.
Dr Lewis said ammunition from the war could still be found in parts of Darwin, especially on the city's beaches.
He said discoveries were becoming rarer over time.
It's been the case that the Northern Territory, after the troops left and the civilians came back in the 1950s, has become common across Darwin suburbs, and it's become less common over the years as they've been found, he said.
There is still a lot of the Northern Territory that has yet to be explored or developed, so you're going to find them for some decades to come. Divers had found spent rifle cartridges and several rounds of bullets at the survey site, even before the suspected ordnance was found on Monday.
A 250 m wide exclusion zone was set up around the object earlier this week, with harbour users warned to steer clear.
The zone is marked by yellow buoys that will be illuminated by lights at night, and warning signs will be erected at several nearby boat ramps in the coming days.
The zone falls within an area used by commercial shipping operators, though the channel remains open as well as recreational fishers trawling for barramundi.
The Amateur Fishermen's Association of Northern Territory AFANT chief executive David Ciaravolo urged recreational fishers to be aware of the zone and stay clear.
He said there should be a good level of awareness in combination with the notices on social media, the notices to mariners and the notices at the boat ramps.
The compliance with the exclusion zone has been good so far, according to DIPL, who thanked the public for their adherence.
A spokeswoman said any harbour users who didn't heed the zone could be issued a fine.