Six candidates are vying for the South Australian seat of Bragg as voters go to the polls in a by-election forced by the resignation of former deputy premier Vickie Chapman.
But the result of Saturday's ballot may not be known until next week, with more than a third of voters casting pre-poll or applying for a postal vote.
As of Friday, more than 4400 voters had cast an early ballot and more than 4700 had asked to vote by post.
These ballots will not be counted until Monday, and could have a significant bearing on the outcome.
Independent candidate Neil Aitchison has top spot on the Bragg ballot paper, with Labor's Alice Rolls in second place and Liberal Jack Batty third.
The Greens, Liberal Democrats and Family First are also contesting the eastern suburbs seat which is likely to be retained by the Liberals despite a big swing against Ms Chapman at this year's state election.
She held Bragg with a margin of 16.6 per cent at the 2018 state poll, but that was slashed to 8.2 per cent in March.
If the Liberals hold the seat, the opposition will have 16 in parliament's 47-seat House of Assembly.
The Labor government holds power with 27 seats with the remaining four held by independents.
More than 25,000 people are enrolled to vote in Bragg where special arrangements are in place for anyone who contracted COVID-19 after the June 30 cut-off for postal applications.
"We do not want them to leave home to vote, but we want to provide them with the opportunity," Electoral Commission spokesman James Trebilcock said.
"If they call the commission we can work with them to ensure they can vote."
At polling booths, officials will be required to be fully vaccinated and wear masks with hygiene officers in place at each location.
Australian Associated Press
Sign up for our newsletter to stay up to date.