A by-election in Perth’s southern suburbs on Saturday has the potential to deliver an unprecedented defeat for the Australian Labor Party, as One Nation vies to capture the traditionally safe seat of Secret Harbour. The outcome could have significant repercussions for both major parties nationwide.

Labor’s Georgia Tree is contesting the seat left vacant by veteran minister Paul Papalia. However, insiders from both Labor and the Liberal Party increasingly expect One Nation candidate Luke Herdegen to secure the win. A victory for Herdegen would mark the first instance of Labor losing a seat directly to One Nation since the party’s resurgence under Pauline Hanson.

While One Nation secured four seats in the South Australian state election in March, those were either held or notionally held by the Liberal Party; Labor did not contest the recent Farrar by-election in the Northern Territory, where One Nation’s David Farley emerged victorious. Should Herdegen prevail in Secret Harbour, it would be a milestone defeat for Labor, highlighting the party’s vulnerability even in electorates previously considered secure.

Western Australia Premier Roger Cook has described the by-election campaign as “the fight of our lives,” warnings that have gained renewed weight as support for One Nation grows. Federal political matters have heavily influenced the contest, with voters often referencing Senator Hanson and Prime Minister Anthony Albanese during candidate interactions.

Cook acknowledged the challenges governments face in by-elections and attributed some difficulties to the rising popularity of One Nation on the east coast. He also pointed to a perceived "toxic coalition" between state Opposition Leader Basil Zempilas and Hanson’s party, which has added complexity to the contest.

A One Nation victory would prompt debate about which major party suffers the greater loss. For Labor, it would signal a break in their longstanding dominance in certain Western Australian districts. The party's substantial majority in the state lower house—holding 46 of 59 seats—could be shaken as backbenchers grow increasingly concerned about their electoral prospects. Surrounding districts, including Premier Cook’s own seat of Kwinana, share similar demographics, raising broader questions about Labor’s hold on these areas.

For the Liberals, a One Nation win combined with a potential third-place finish for their candidate Ryan Robertson would deepen uncertainty over the party’s strategy to regain government. Opposition Leader Zempilas has maintained that any loss for Labor is ultimately a positive outcome for the Liberals, even if that means ceding ground to One Nation. “There’s the very real prospect of an incumbent Labor government being thrown out in a seat that they have always held,” he said, portraying such an outcome as a victory for voters frustrated with the current government.

The by-election has also exposed internal tensions within the Liberal Party. Federal MP Andrew Hastie, whose electorate of Canning overlaps with Secret Harbour, has publicly criticized the state Liberals for what he perceives as a lack of vigorous opposition not only to Labor but also to One Nation. With Canning seen as a key target for One Nation, a win by Herdegen could bolster the party’s ambitions for the next federal election.