Votes were being rechecked on Monday following Saturday's dual Victorian state by-elections after Labor suffered a huge swing against it in Werribee and the Liberals claimed victory in Prahran.
Labor has held Werribee in Melbourne's outer west since 1979, but the party's primary vote was down 16.7 per cent at last count.
It is still expected to cling on to the seat after the return of postal votes, with Labor candidate John Lister leading the Liberals' Steve Murphy 50.55 per cent to 49.45 per cent on a two-candidate-preferred basis.
A final result is expected to be announced by Friday.
The swing in Werribee was not a shock given flagging support for state Labor in lead-up polls, election analyst and blogger Ben Raue said.
The preliminary results continued the trend of voters deserting the major parties - Labor and Liberal candidates each received less than 30 per cent of primary votes - in favour of independents and minor parties.
Federal Labor politicians have downplayed the 10 per cent swing in Werribee ahead of the national election that must be held by May 17.
Mr Raue said it was another piece of the puzzle, along with recent polling, that showed federal Labor was also on the nose.
"There is evidence that they're doing badly," he told AAP.
Political scientist Zareh Ghazarian also cautioned the federal coalition not to read too much into either by-election, saying they were mainly fought on local and state issues, with voters passing judgement on a decade-old government.
"Labor in Victoria appeared to be unbeatable," the Monash University senior lecturer told AAP.
"That era of invincibility seems to be wearing off and that will rub off a bit on the federal party.
"But I wouldn't expect these sorts of swings to be apparent at a general election because that's not really what the opinion polls are telling us is going on at a national level."
Voters turned on Victorian Labor but did so knowing they could fire a "warning shot" without changing the government, Dr Ghazarian said.
"If these swings were recorded across many seats, I think there would be at least 20 seats in doubt," he said.
"What the premier has not really signalled in the wake of the Werribee by-election is what will actually change."
The swing was unacceptable and Labor had heard the message "loud and clear", deputy premier Ben Carroll said.
The Werribee result has prompted renewed speculation the government could pause the $34.5 billion eastern section of the Suburban Rail Loop to stem the electoral bleeding.
But Mr Carroll and Premier Jacinta Allan were on a unity ticket on Monday, indicating it was full steam ahead for the contentious mega-building project in response to repeated questioning.
Liberal candidate Rachel Westaway claimed victory in Prahran, unseating the Greens for the first time since 2014.
The Prahran by-election was triggered by the resignation of Sam Hibbins after he revealed an affair with a staffer.
The Greens blamed the result on absentee votes not being allowed and losing preferences after Labor opted not to run a candidate.
Mr Raue said the result went deeper than those factors and indicated Victoria was on a swing to the right.
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