Labor was easily returned to power in Saturday night's South Australian election with an increased and overwhelming majority, giving the party a stranglehold on the state's lower house.
But a 19 per cent swing to One Nation has destroyed the Liberals' primary vote, putting the minor party ahead of its conservative rival on first preferences.
One Nation candidates were leading the primary vote in the lower-house seats of Hammond, Mackillop and Ngadjuri, with the as yet unclear flow of preferences to determine the results.
As of Sunday morning, Labor was on track to win at least 32 of South Australia's 47 lower-house seats, exceeding its previous count of 29.
The Liberals were likely to secure four seats with a handful more too close to call.
Premier Peter Malinauskas, who met with the state's governor on Sunday, acknowledged the One Nation surge could reshape Australian politics after the right-wing party's best election result since the 1998 Queensland state poll.
"The moment you start assuming the upper limit on any political opponent is the moment your primary vote starts going down," he told ABC's Insiders program.
"We've been able to preserve (our vote), but I don't think we should take too much comfort from that.
"The idea of safe seats, that's over, the idea of long-standing, intergenerational party loyalty amongst families is out the window."
The Liberals' primary vote collapsed with a 16.9 per cent swing away from the party with more than 50 per cent of ballots counted.
Most of One Nation's gains had come at the expense of the Liberals, Mr Malinauskas said, but he conceded some Labor voters had also shifted to supporting the populist party.
It could take days to determine how many lower-house seats One Nation won, however the premier was quick to lash his new parliamentary rivals.
"What you see is a lot of rhetoric … and almost zero policy; they made a virtue out of almost not offering a policy proposition to the people of our state," he said.
One Nation leader Pauline Hanson pointed to state head Cory Bernardi's success in winning his upper-house seat, adding the party would be going hard at November's Victorian state election.
"There's a movement, there's an undercurrent and it's people saying we've had a gutful," she told Sky News.
"We want our country back. We want to have a voice."
Mr Bernardi said an earthquake had rattled the foundations of traditional party politics in the state.
Liberal leader Ashton Hurn retained her seat in the Barossa Valley and will remain in her role.
Federal Liberal senator Anne Ruston said the party had been sent a clear and "sobering" message and it couldn't win by trying to shift more to the right or left.
"The Liberal Party has got a lot of work to do to rebuild the trust of Australians, I absolutely believe that we can do that," she said.
Federal frontbencher Chris Bowen said Mr Malinauskas had cemented his place as a "Labor great" with the size of his landslide win and his role in advocating for national reforms, including Australia's world-first social media age restrictions.
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