Premier Peter Malinauskas and Opposition Leader David Speirs both predicted a tight finish in South Australia's most marginal electorate, with the possibility of no clear result on Saturday night.
The contest was forced by the resignation of former Liberal premier Steven Marshall after 14 years in the seat following Labor's surprisingly emphatic victory at the 2022 state election.
The electorate, named after revered Labor premier and hot pants aficionado Don Dunstan, who held the seat from 1953 to 1979, has swung between the major parties since his retirement.
The by-election comes at the midway point of Mr Malinauskas' first term and represents his first major electoral test.
A result against the government would not be out of the ordinary but a large swing, such as the one suffered by the Queensland government in last weekend's Inala by-election, could be cause for concern, election analyst Ben Raue told AAP.
Failed election promises, rising ambulance ramping and cost-of-living pressures have soured the electorate's initial goodwill and a big swing against the government would pile further pressure on its leader.
What role that dissatisfaction will play in the affluent inner-east electorate, where voters are relatively less exposed to issues such as public health under-resourcing, remains to be seen.
South Australian voter Saki has put his money on a Labor win. (Jacob Shteyman/AAP PHOTOS)
At Norwood Primary School polling booth, where sausage sangas still cost a democratic $3.50, Saki was tucking into his third of the day.
Despite counting the cost of living as his biggest concern, the 60-year-old was pledging his vote for Labor.
"It's really a struggle right now but I think Labor's gonna do a much better job than the Liberals," he told AAP.
History points to a Liberal win.
The last time a South Australian government won a seat off the opposition in a by-election was more than a century ago.
But any swing against them would put the seat in serious doubt and raise further questions about Mr Speirs' leadership, with rumours swirling that the Liberals were priming first-term MP Ashton Hurn for the role.
Labor's Cressida O'Hanlon enjoyed the support of her family before voting. (Jacob Shteyman/AAP PHOTOS)
The Liberals hold Dunstan on a wafer-thin 0.5 per cent margin after business mediator Cressida O'Hanlon, who is once again contesting the seat for Labor, enjoyed a 6.9 per cent swing towards her at the last election.
"Over the last three years, I have knocked on over 15,000 doors and I've done that because I am truly local, because I know this community and because I love this community," she said in an eleventh-hour pitch to voters.
The Liberals have selected a "star candidate" of their own in the form of lawyer Anna Finizio.
Like Ms O'Hanlon, Dr Finizio said she would advocate for local issues, including heritage protection, traffic abatement and preserving tree canopies if elected.
Liberal candidate Anna Finizio cast her ballot in St Peters (Jacob Shteyman/AAP PHOTOS)
Despite the last-minute focus on policy, the campaign has been defined by personal attacks.
The Liberals have accused Ms O'Hanlon of abusing her position as a political staffer to help her husband secure meetings with government ministers - a claim she has denied.
Meanwhile, Labor has alleged Dr Finizio lied about her history as a company director on her candidate declaration form.
Voter disillusionment with the major parties over the perceived mud-slinging could work in the favour of Greens candidate Katie McCusker, who hopes to increase her party's share of the vote from 13.7 per cent.
More than 7000 of an expected 27,000 votes were entered early and won't be counted until Monday, meaning it could be a nervous wait into next week for the outcome.
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