Former Liberal deputy leader and tennis ace Sam Groth resigned from Victoria's parliament on Friday, triggering a by-election in his seat of Nepean on Melbourne's Mornington Peninsula.
Voters in the electorate will be forced to go to the polls twice in one year after the by-election date was announced for May 2 before the general election on November 28.
The election in the safe Liberal seat will be an important indicator on the opposition's standing, as it enjoys a positive voter attitude compared to Labor in opinion polls, says Monash University politics expert Zareh Ghazarian.
"A strong win in Nepean would consolidate that sense, but a loss or decline in the party's vote would ... raise questions about its policy and personnel settings," he told AAP.
Labor have yet to announce a candidate but could skip it altogether as the party did during the Prahran by-election to preserve resources for the November campaign.
If it does contest the election, Dr Ghazarian said a poor result close to the general election could destabilise the party.
By-elections were super-focused on local personalities and issues, the political scientist said.
"It will give those who want to give the major parties a kick up the backside an opportunity to do so," he said.
Dr Ghazarian said opinion polls showed voters were not too happy with the major parties, and also suggested a rise in support for One Nation.
"(The by-election is) going to test the extent to which people are willing to support the major parties ... or whether they are now willing to support new challengers," he said.
"Depending on the result, we might get different narratives emerging in the lead up to the election, and that might potentially give momentum to or take momentum away from the established parties."
Over the electorate's 24-year history, Labor has held Nepean, which has one of the oldest median ages of any state seat, for one term following the party's landslide 2018 election.
That MP, Chris Brayne, who Mr Groth defeated in 2022, said the Liberal MP's resignation was an "entirely predictable outcome".
Mr Brayne expressed frustration for locals who have to vote twice in a year, urging all parties to redouble their efforts to select good-quality candidates.
"We need candidates who are in it for the long haul, who will weather the good and bad days," he said.
Mornington Peninsula Shire mayor Anthony Marsh has indicated his interest in running for the Liberals.
The coalition will need to net at least another 16 seats in November to form majority government and end Labor's reign after 12 years.
