Farmers are on high alert after 78 nests were discovered at Swartz Barracks at Oakey, west of Brisbane.
The pest's arrival in the basin is of grave concern, given warnings fire ants could harness river flows to quickly invade new parts of Australia.
The Invasive Species Council believes the Oakey infestation could be years old, given how many nests there are.
Fire ants campaigner Reece Pianta is waiting on genetic testing that will confirm if the outbreak is linked to the long-standing fire ant battleground centred on Brisbane and the Gold Coast.
He says there's lots of defence land in and around southeast Queensland's fire ant biosecurity zones.
But the national eradication program, delivered by the state government, "appears to be finding it hard to respond to fire ants detected on federal land, including defence land".
He says it's unclear if active suppression is being carried out at defence sites, as it is elsewhere.
"This latest detection shows that is a gap in Australia's fire ant response."
Ken Cunliffe of farming lobby group Agforce is calling for public land to be assessed for fire ants. (HANDOUT/AGFORCE QUEENSLAND)
Ken Cunliffe from Agforce, a farming lobby group in Queensland, is also convinced the ants have been at Oakey for years and is pushing for all public land to be properly assessed and audited.
"It's not just military facilities it's also national parks ... all the other public facilities, main roads, all the road verges, they really need constant attention."
Farmers south of the border are "extremely fearful".
"If you own a property that's bordering the Murray Darling, you're now going to have a major concern that this has the potential to have a huge level of impact," NSW farmer Ian McColl has told AAP.
Serious concerns about defence land were raised in a 2021 review of the fire ant eradication program.
That review was handed to the government when Scott Morrison was still prime minister but it wasn't released for another two years, despite making a series of urgent recommendations.
Among them was the need to actively suppress fire ants on defence land.
The chair of that review recently told an inquiry she's still not sure if fire ants are being properly managed at defence sites that are outside the jurisdiction of the eradication program.
Dr Helen Scott-Orr recalled looking at a map of the Brisbane infestation.
"There was a high build-up of infestations in Brisbane and then there was a big hole in the middle," the former Australian Inspector-General of Biosecurity told the inquiry into the fire ants response.Â
When she asked what the hole was, she was told it was the Greenbank military reserve.
"It looked as if there were probably lots of fire ants on that military reserve, but this was not in the jurisdiction of the national program. It was not in the jurisdiction of the Queensland state government to go onto that land, and therefore no-one was doing it."
The defence department has told AAP it's working with the eradication program to manage the recent detection at Swartz Barracks, and minimise the risk of spread.
The national eradication program says it's prioritising eradication activities in Oakey.
"So far the fire ant nests detected in Oakey are contained to one infected property," a spokesperson says.
"Intensive surveillance and treatment continues this week.
"Compliance and tracing investigations are also continuing to help determine how the ants arrived in the area."
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