Voters in the coastal NSW electorate of Kiama backed Labor candidate Katelin McInerney to replace former independent MP Gareth Ward, who was forced to resign after being convicted of sexual and indecent assault.
The Liberals' underwhelming result could jolt NSW Opposition Leader Mark Speakman's grip on the party, while Premier Chris Minns notched up an electoral win.
It's the first time in three decades a seat in a by-election has gone to the government.
Mr Speakman vowed to lead the coalition to the 2027 state election and said the Liberal Party's federal turmoil hindered his campaign.
Federal Opposition Leader Sussan Ley dumped Jacinta Nampijinpa Price from her shadow cabinet last week after she failed to back her leadership following controversial remarks about migration.
"That public airing of differences does not help," Mr Speakman said.
"The Liberal Party has to present itself as united, focused and disciplined, addressing the reality of modern Australia.
"This was a by-election held in extraordinary circumstances … my focus now is on making sure we are match-fit for 2027."
Mr Minns was quick to downplay the significance of the result given the "grave and serious" circumstances, and said Labor would have lost by-elections in other parts of NSW.
"It was very advantageous to the Labor Party that a by-election fell due to the incumbent member of parliament sitting in jail," he said.
"The voters of NSW expect us to make major inroads and real progress on the challenges they're facing in their community, and if we don't make real progress, we will not win the 2027 election."
Mr Minns said he was concerned his MPs would misread the Kiama result and think they had won the next statewide count "without even fighting for it".
He acknowledged a strong vote for minor parties and independents.
About a third of voters cast their ballots for candidates outside the two major parties.
Election analyst Ben Raue called the win for Labor two hours after polls closed, characterising it as a comfortable victory.
He noted later Labor's two-party preferred vote was about 60 per cent, with a 10.7 per cent swing. Labor's primary vote swing was about three per cent.
The result was widely forecast after Ms McInerney fell less than 700 votes short against Ward in 2023.
The win takes Labor to 46 seats in the Legislative Assembly, one shy of a full majority.
Ward resigned in August after he was convicted for sexual and indecent assault. He is due to be sentenced next week.
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