The NSW government will deploy "one of the world's strongest mice-killing chemicals" to combat the state's plague - but there are concerns it could kill other animals too.
Country towns have been struggling since last year with an infestation that has ruined crops, damaged tonnes of stored hay and grain, infiltrated homes and caused millions of dollars of damage.
The NSW government said on Thursday it had secured 5000 litres of the anti-coagulant bromadiolone - enough to treat about 95 tonnes of grain - and would provide it for free once federal authorities approve its use.
The chemical is able to kill mice within 24 hours of its consumption.
The measure forms part of a $50 million government package announced last week to help farmers and regional towns suppress mice numbers.
But farmers and environmentalists say there is a question mark over the chemical's safety.
Bromadiolone is highly lethal in just one dose but takes a while to kill, meaning rodents are likely to consume a highly toxic amount as they keep coming back.
That could mean the poison kills native birds and other wildlife who eat the doomed mice, BirdLife Australia said on Thursday.
"To see a mass roll out of this dangerous chemical being proposed now is extremely concerning," BirdLife's Holly Parsons said.
"There are other, less harmful chemicals available."
The NSW Farmers' Association is also warning of the possible effect on farm dogs, piggeries and other animals.
"There are a serious number of question marks but we trust the science to get it right," Farmers NSW Vice President Xavier Martin said.
But the need to wait for regulatory approval from the Australian Pesticides and Veterinary Medicines Authority means it's too little, too late, he said.
Mr Martin, a grain farmer from the Liverpool Plains, said the window is closing to sow crops like canola which sustain the state's bee population.
Farmers are now deciding against planting it because the mice won't be killed quickly enough.
"This is a right-now problem," he said. "We've effectively got ten days to get this crop in the ground."
He added that 5000 litres is not enough to treat even a quarter of the state's winter crop paddocks.
"The other three quarters are untreated and of course mice will immediately spread just like COVID on a cruise ship," he said. "It's just impractical nonsense."
NSW Farmers is calling for primary producers to get a 50 per cent rebate on zinc phosphide, an alternative poison, instead.
The group conveyed their displeasure in a Thursday meeting with NSW Agriculture Minister Adam Marshall.
Mr Marshall said in a statement on Thursday that NSW had secured a local supply of the chemical which means it's "ready to roll".
"Experts will treat growers' grain with bromadiolone completely free of charge to build a mice-free fortress to protect paddocks."
Mr Marshall said the chemical should be used in conjunction with baiting.
NSW Premier Gladys Berejiklian on Thursday said the government was working closely with experts in the area to deal with the mice as safely and quickly as possible.
"It's a dire situation and we're trying to source the best way of dealing with the plague, supporting farmers and communities," she said.
Australian Associated Press