Rangers are devastated that a population of black rats has infested a Great Barrier Reef Island declared pest-free just months ago.
The common house mouse was eradicated in August from North West Island, off the central Queensland coast, after an extensive two-year effort by rangers.
Assistant principal ranger Damon Shearer said the team was "extremely disappointed" to find a new infestation of rats.
"Rats can be incredibly devastating on an island like North West," he said.
The pristine coral cay, 75 kilometres north-east of Gladstone, is the largest breeding island for wedge-tailed shearwaters and black noddies on Australia's east coast.
Before the mice were eradicated, they were eating seabird eggs and preying on turtle hatchlings.
"We've got ground-nesting shearwaters [and] black noddies nesting in the trees," Mr Shearer said.
"It's also a turtle nesting island and we're pretty much smack bang in the middle of the season for both turtles and seabird nesting.
"[The rats could have] potentially massive impacts on the animals out there."
When were rats detected?
Mr Shearer said a possible rat sighting was reported to the Queensland Parks and Wildlife Service (QPWS) on December 2.
Rangers subsequently installed 21 ink track tunnels, two trail cameras, and 60 baited rodent stations, before checking them on December 9.
"10 of those 21 [tunnels] showed up with rodent tracks," Mr Shearer said.
The trail cameras also detected rats.
The campground has a capacity of 150, which Mr Shearer said was often full during peak times.
Barges and privately owned boats also visit the island.
"These black rats have turned up basically between August and December at some stage," Mr Shearer said.
A full control program is underway to understand the size of the population with monitoring, extra rangers, more bait stations, tracking, and active trapping.
How did they arrive on the island?
Griffith University ecologist Guy Castley said, although rats could swim, the risk of the animals making it to North West Island was low, considering how far the island was from the mainland.
He said the most likely scenario was the pests hitched a ride.
"For example, somewhere they've got onto a vessel that's docked on the mainland, that vessel goes out to the island and then the rat jumps off at the island and swims to shore," Dr Castley said.
"[Mice] were eradicated just previously, rats potentially pose more of a risk because they're bigger than mice [and] they breed faster than mice."
Dr Castley said the management of pests on islands was an ongoing process, requiring monitoring and vigilance.
"It can be quite demoralising for the people who are trying to achieve those conversation outcomes," he said.
"It is a long process to have an island declared pest-free and then have something that comes up again, or is detected shortly afterwards.
"Having people detect them early allows us a window of opportunity to get on top of it."
Rangers plea to visitors
Mr Shearer urged visitors to check clothing, footwear, camping gear, vehicles, and boats for pests, weeds, and contaminants before arriving.
Soil, seeds, plants, insects, and animals, such as rats, mice, lizards, and toads all pose a danger to the island's flora and fauna.
"It's a lot of expense — a lot of time went into the planning of the eradication," Mr Shearer said.
Having to repeat a full-scale eradication, including helicopter aerial baiting, would be a huge blow to rangers, he said.
"We really need the public's help," Mr Shearer said.
"People cannot take shortcuts with biosecurity measures, as rats and mice are extremely cunning and can stow away in small places and slip out of sight without being seen."
Anyone who sees a rat on North West Island should report it to the Department of Environment and Science on 1300 130 372.
