Five days before the Wentworth byelection, when Liberal candidate Dave Sharma was starting to claw back support in the seat Prime Minister Scott Morrison was desperate to save, Facebook and Twitter lit up with a warning: if local doctor Kerryn Phelps doesn't come second, the Liberals win.

Pushed by the GetUp campaign group and amplified by Phelps and her supporters, the tactical-voting message may have marked a turning point in Australian politics: the moment social media played a decisive role in a major election.