The South Coast's Currowan fire is expected to tear through a former military artillery range, potentially causing massive explosions in an area inaccessible to fire crews.
Tianjara range, about 50 kilometres southwest of Nowra, was primarily used for artillery training from World War II until the 1980s.
But its expansive grounds were also blasted by grenades, mortars, and anti-tank weapons, a Department of Defence spokeswoman said.
Historian Bob Snedden, who wrote the book on the history of the range, which is called Tianjara, said the amount of unexploded ordnance, or UXO, on the site couldn't be measured.
"The big problem is, there's very little in the way of records that are kept of the place," he said.
"And there's no way in the way in the world, due to the terrain ... that the amount of unexploded ordnance [could] ever be properly quantified."
His view is backed by the Department of Defence. Its spokeswoman said it was "impracticable" to sweep Tianjara's 25,000 hectares of contaminated land for UXO because of environmental impacts.
The former range also presents accessibility issues for fire crews. NSW Rural Fire Service public liason officer, Phil Paterson, said the Currowan fire was very likely to reach the range unless the area got a significant rain event.
Instead of tackling the fire if it hit, crews would be forced to wait it out. Waterbombing could set off explosives at the range, and pilots couldn't fly over it at a height of less than 1000 metres.
"There's pretty big pieces of nasty business in there," Mr Paterson said.
"Those air tankers get down as low as 300 feet, or lower, so they could be well within range of experiencing an explosion."
Tianjara fell within wide fire containment lines, but if tankers couldn't bomb it, it was vital firefighters stayed out, Mr Paterson said.
Terry Barrett was NSW National Parks and Wildlife Service's superintendent for Morton National Park from the early 1980s until the 1990s. That was about the time Tianjara became part of the park, and the service assumed responsibility for the range.
Mr Barrett said the density of vegetation in Tianjara was in itself a massive fire risk - without the additional threat of explosives.
"It's more than six feet or a couple of metres high, and dense as blazes in some parts," Mr Barrett said.
"It will go off like a cracker."
He said fire moving through the range posed a threat to properties only a few kilometres away, but Mr Paterson said a blaze would be limited to the national park.
Mr Barrett said a major fire hit Tianjara range in the 1980s - he took on the role of deputy emergency fire control officer when it happened - but fires had became much worse since then.
"[We're in] a situation that has no precedents, I can tell you now," he said.
"It's got to be something that's changing the dynamics of the climate and as far as I'm concerned, we're in a global warming emergency."
Mr Paterson said Morton National Park would remain closed until further notice. Normally, some designated walking tracks in Tianjara were open to the public. "The risk [of explosions] is real," he said.
A spokesman for NSW National Parks and Wildlife Service said it had a fire management strategy for Morton National Park in place. It was working with the NSW Rural Fire Service and other agencies to manage the fire risk in Tianjara.