Nepean by-election: Liberal infighting and One Nation challenge test safe seat

Key facts

• Nepean by-election takes place on Saturday, March 25, 2023.
• The seat covers affluent bayside communities including Sorrento, Portsea, Dromana, Rosebud, and McCrae.
• Liberals won more than two-thirds of the two-party preferred vote in Nepean in 2022.
• No Labor candidate is contesting the by-election.
• Former deputy Liberal leader Sam Groth resigned suddenly, triggering the by-election.
• Anthony Marsh, Mornington Peninsula Mayor and recent Liberal recruit, is the Liberal candidate.
• One Nation candidate Darren Hercus, a civil engineer, is running.
• Victorian Liberal leader Jess Wilson needs to retain all 29 Liberal seats and gain 16 more to win the November state election.

A by-election that should be a formality becomes a test of Liberal unity

On Saturday, voters in the Mornington Peninsula seat of Nepean head to the polls for a by-election that, under normal circumstances, would be a comfortable hold for the Victorian Liberals. The party has held the seat for all but one term in the past 40 years, and with no Labor candidate on the ballot, the path to victory appears clear. Yet these are not normal times for the Victorian Liberal Party. The contest has become a barometer of internal cohesion and electoral viability ahead of the November state election, where the party must overcome a 16-seat deficit to unseat Premier Jacinta Allan's Labor government.

Liberal infighting and a sudden resignation set the stage

The by-election was triggered by the abrupt resignation of Sam Groth, the former deputy Liberal leader and Nepean MP. His departure, combined with local unrest over the preselection of Mornington Peninsula Mayor Anthony Marsh—who joined the party only weeks before—has exposed deep fractures within the Victorian Liberal Party. Marsh's candidacy has not healed those wounds. The party has been at war with itself, a conflict compounded by the federal Coalition's own existential crisis. The internal strife threatens to dampen turnout and push moderate Liberal voters toward alternatives.

One Nation sees an opportunity in Nepean's demographic mix

While the Liberals remain favourites, Nepean's demographics offer openings for challengers. The seat includes wealthy bayside communities such as Sorrento and Portsea, where the Liberals won more than two-thirds of the two-party preferred vote in 2022. But it also takes in Dromana, Rosebud, and McCrae, areas with lower incomes where Labor was more competitive. One Nation candidate Darren Hercus, a civil engineer who runs a joist and truss manufacturing business in Dromana, is capitalising on discontent. One Nation's performance in South Australia's March state election demonstrated the party's growing appeal in non-metropolitan areas and among older voters—a demographic well represented on the peninsula.

The stakes for Liberal leader Jess Wilson

For Liberal leader Jess Wilson, the Nepean by-election is more than a single seat; it is a test of her leadership and the party's electoral machinery. Wilson needs to retain all 29 Liberal seats and pick up 16 more to defeat Jacinta Allan in November—a massive swing by historical standards, even with Allan's current unpopularity in opinion polls. A poor showing on Saturday, even if the Liberals win, would signal that the party remains unable to consolidate its base or expand its appeal. Conversely, a strong result could provide momentum and a narrative of renewal.

What the result will reveal about Victoria's political landscape

The by-election will offer the first concrete electoral data since the 2022 state election, providing insight into voter sentiment after a year of Liberal infighting and a cost-of-living crisis. The absence of a Labor candidate means the contest is essentially a two-way race between the Liberals and One Nation, with minor parties and independents also on the ballot. Analysts will watch the swing against the Liberals and the One Nation vote share closely. A strong One Nation performance could signal that the party is becoming a permanent force in Victorian politics, siphoning votes from the Liberals and complicating Wilson's path to government.

A by-election that foreshadows November's battle

Saturday's vote in Nepean is a dress rehearsal for the November state election. The result will shape perceptions of Liberal viability and One Nation's trajectory, while Labor watches from the sidelines, unopposed but not uninterested. For the Liberals, a win is expected, but the margin and the narrative around it will matter far more. In a political world turned upside down, a safe seat has become anything but.

The bottom line

• The Nepean by-election is a critical test for the Victorian Liberals, who face internal division and a challenge from One Nation despite the absence of a Labor candidate.
• Liberal leader Jess Wilson needs to retain all 29 Liberal seats and gain 16 more to win the November state election—a historically large swing.
• One Nation candidate Darren Hercus is targeting disaffected Liberal voters in lower-income areas of the seat, building on the party's recent success in South Australia.
• The by-election was triggered by the sudden resignation of former deputy Liberal leader Sam Groth, and local unrest over the preselection of Anthony Marsh has deepened party fractures.
• The result will provide key data on voter sentiment ahead of November, particularly the strength of One Nation and the Liberals' ability to unite their base.