Thirty years ago, this highly flammable weed had not made it into Litchfield National Park at all.
Today, gamba grass has taken over about 20 per cent of the park, which is one of the Northern Territory's most treasured tourist attractions.
And experts say it is now virtually impossible to completely eradicate.
For Litchfield park rangers, it is a major threat to the park's biodiversity because it increases the likelihood of yearly burn-offs turning into devastating wildfires.
"Gamba grass is really dangerous to people's safety, but also to the [park's] biodiversity because its fuel loading is so much higher than the native spear grasses, or the native grasses in general," Senior Park Ranger Sam Washusen said.
"The radiant heat can really displace a lot of other vegetation — it can burn the trees and prevent them from going to seed.
"And it will add or replace the open woodland with a monoculture of gamba grass if we don't get onto it quick enough."
In a recently released report, ecologists from the National Environmental Science Program (NESP) and Charles Darwin University (CDU) recommended the NT government urgently expand its current gamba eradication zone.
If it isn't expanded, the report found gamba grass will replace the existing ecosystems across about 30 per cent of the park by 2032.
Eradication 'almost unimaginable'
Mr Washesun said the problem is so advanced that it's now virtually impossible to completely eradicate gamba from the park.
"We're just focusing on what we can within the eradication zone at the moment because this is what our resources are allowing us to do at this point in time," he said.
"If you're going from managing 600 hectares of gamba grass to 30,000 hectares of gamba grass ... if you're talking about across the whole park, it's almost unimaginable the amount of extra effort you'd have to put into it."
He said he was supportive of expanding the current eradication zone and working towards "reclaiming areas over time".
However, focusing only on specific areas comes at a cost.
"The gamba has been getting thicker over the years because we're concentrating our efforts on the eradication zone," Mr Washusen said.
"And we are ending up with some areas of the park, especially up in the north-east of the park and right down in the south-west of the park, we're getting infestations which are increasing in density."
Mr Washesun said that yearly dry season burn-offs could catch on in those remote infestations, and create wildfires that speed up the demise of native woodlands.
"There are some areas that you can see on the way to the park ... you're seeing the high intensity fires where the trees are not being recruited like they had been in the past," he said.
"Similar things will be happening out on the park as well in those areas where we're unable to put the concentrated effort into trying to reduce the amount of gamba density there."
Carbon business in jeopardy
The Thamarrurr Rangers, who are based in the remote community of Wadeye, manage 18,000 sq km of land across the western Top End.
The group accrues carbon credits by preventing late season bushfires with early season burn-offs as a part of a carbon business established in 2020.
But they say the kinds of wildfires created by the accidental burning of gamba grass generates enough C02 to wipe out the emissions saved through through this practice.
"If you don't do something about it, and if someone lights it up it just goes like 'that' because it's very thick grass. They don't know its gamba," Ranger Christine Tchemjiri said.
Fellow ranger Louis Boyle-Bryant said the spread of gamba is a serious threat to the group's livelihood.
"You're looking at losing potentially a lot of revenue over time," he said.
"As gamba grass moves in, if you're looking at losing tens or thousands of hectares over a longer time period, you're looking at losing a significant portion of your carbon business."
Cost of eradication doubles since 2014
The NT government's 2014 weed management plan placed Litchfield National Park within the boundaries of a gamba "management" zone, west of the Mary river and north of Daly River, where eradication was not deemed feasible.
Two years later, a small gamba eradication zone within the wider management zone, spanning 600 hectares around the park's highest profile sites — Buley Rockhole and Florence Falls — was established.
According to the NESP and CDU research, lead by ecologist Dr. Natalie Rossiter-Rachor, neither the 2014 survey nor the 2016 conservation strategy was funded to implement an intensive eradication program.
That inaction, she said, means the cost of fully eradicating gamba grass in those areas is now estimated to be about $1 million over the next five years — double what it was in 2014.
"But we would actually recommend that there's a much larger eradication zone created to cover both the sandstone plateaus in the park," she said.
"That is going to cost $7 million over five years to protect the visitor assets and biodiversity assets in this area.
"It's critical that we do this now before the cost of this management doubles."
The NT government has not yet confirmed whether or not they will adopt the recommendations from the NESP and CDU report.
However, a spokesperson for the NT Parks and Wildlife department said more than $1.2 million had been spent on on gamba grass management in Litchfield since 2014.
They also said the government spends $6.5 million annually on weed management, including gamba grass.
