A Victorian state poll of 1274 voters, conducted by DemosAU/PremierNational from February 1 to 10, put One Nation's primary vote at 21 per cent.
The result isn't quite as strong as the 26 per cent watermark indicated in recent national polling.
But it represents a 20.7 per cent swing from the 2022 Victorian election across a state that ex-prime minister John Howard once described as the "Massachusetts of Australia" for its left-leaning tendencies.
Labor's primary vote has crumbled to 23 per cent, while the coalition fared only mildly better at 29 per cent after the Liberals switched state leaders from Brad Battin to Jess Wilson in November.
While the coalition holds a 53 per cent to 47 per cent two-party-preferred lead, its primary vote has slumped eight points since the last DemosAU poll in October.
If the results were replicated at the election, DemosAU head of research George Hasanakos said a change of government would be likely.
"In Victoria we see a government on the nose and a struggling Liberal party brand federally, resulting in many voters sending their vote to One Nation," he said.
"Whether this is a permanent shift or voters are merely parking their vote remains to be seen."
In 2022, dairy farmer turned upper house MP Rikkie-Lee Tyrrell became One Nation's first ever representative elected to Victorian parliament.
The former fringe party is recruiting to field candidates in all 88 lower house seats and eight upper house regions at the state election on November 28.
Mr Hasanakos flagged a "wildcard" possibility of One Nation holding the balance of power in the lower house.
"With 20 per cent of the vote, we can expect to see One Nation winning seats, especially as their vote is concentrated in regional areas, where they are outpolling both major parties," he said.
Opposition Leader Jess Wilson dismissed the significance of the poll and wouldn't be drawn on the coalition leaking votes to One Nation.
"Polls, you know, they come and go," she told reporters on Sunday.
"Victorians understand this state is headed in the wrong direction ... they want change and I'm excited to offer them that over the next 10 months."
Ms Wilson's former deputy Sam Groth tendered his resignation from parliament on Friday, setting up a by-election in his seat of Nepean on Melbourne's Mornington Peninsula.
Nominations for Liberal preselection remain open and Labor is yet to decide if it will field a candidate.
Even if the Liberals retain Nepean, the Victorian coalition will need to net at least another 16 seats in November to form majority government and end Labor's reign after 12 years.
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