Peter Malinauskas is likely to become the first South Australian premier to gain a seat off the opposition at a by-election in more than a century in a massive show of faith for his government and leadership.
ABC election analyst Antony Green on Saturday night predicted Labor candidate Cressida O'Hanlon was on track to claim the seat with almost half the vote counted, but stopped short of calling it for the 51-year-old.
Ms O'Hanlon had received a 3.4 per cent two-party preferred swing with early votes yet to be counted, eclipsing the Liberals' slim 0.5 per cent margin.
"There is definitely a swing to Labor and if that proceeds through then Labor would win the Dunstan by-election, and that would be a loss of a seat for the opposition in South Australia, but there is more counting to come," Mr Green said.
What was clear on the night was the electorate had delivered a huge swing to the Greens and away from the major parties.
Whether voters were attracted to the party by its promise to extend a tram line to Norwood or if they were simply disillusioned by the mud-slinging of the major parties that had defined the campaign may never be known.
SA Greens Senator Sarah Hanson-Young congratulated her party's candidate Katie McCusker on the result after a commendable 8.8 per cent swing in her favour.
That result was bad news for the Liberals, with Greens' preferences flowing to Labor at almost 90 per cent.
Opposition Leader David Speirs stopped short of conceding defeat but acknowledged the path to victory was difficult.
"The results tonight are not looking that promising for the Liberal Party," he told a room full of volunteers.
"There is a pathway to holding the seat, but it is a pathway which has many barriers and so that's going to be challenging, and for that result, I apologise.
"I am really sorry that it is as difficult as it might be."
Liberal candidate Anna Finizio thanked her team of volunteers for their efforts and told them to walk away with their heads held high, regardless of the result.
With a history-defying loss now on the cards, whispers of a challenge to Mr Speirs' leadership will become louder, with first-term MP Ashton Hurn seen as a long-term prospect in the role.
The mood was much less dour at Labor headquarters, where Mr Malinauskas and his apparent new parliamentary colleague were soaking up the result.
"Cressida and I are both quite taken aback to be in this extraordinary position," Mr Malinauskas said.
"It seems pretty remarkable that we are on the verge potentially of doing something that hasn't happened in South Australia in 116 years."
The contest was forced by the resignation of former Liberal premier Steven Marshall in January following Labor's surprisingly emphatic victory at the 2022 state election.
Mr Marshall held the seat for 14 years and the loss of his appeal no doubt dragged down the Liberal vote in Dunstan.
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 came at the midway point of Mr Malinauskas' first term, at a time when it appeared rising ambulance ramping and cost-of-living pressures were eroding the electorate's initial goodwill.
Both leaders had predicted a tight contest, but the prospect of a swing towards the government after big swings away from the Queensland and federal governments in recent by-elections was an emphatic tick of approval for the premier.
More than 8000 pre-poll votes won't be counted until Monday, meaning a definitive outcome might not be known for a few days.
