If everything goes according to plan for Australia's two most prominent political outsiders, looming elections in Queensland and South Australia could stand as joint watersheds in the decline of Australia's two-party hegemony.

The spectre of One Nation matching or even exceeding its 11-seat haul continues to hang over state politics in Queensland, where the most recent poll had the party's primary vote support nudging 20%, despite its ongoing internal management issues.

Now Nick Xenophon stands poised to unleash his full powers of disruption at the South Australian election in March, after announcing on Friday that he will leave the Senate to take on the eastern Adelaide seat of Hartley for his SA Best party.