The ALP was expected to easily win Saturday's election, but all eyes were on One Nation and whether it could meet polling expectations.
With 2.1 per cent of ballots counted, Labor held 45.7 per cent of the vote, with the Liberals on 15.6 per cent and One Nation sitting on 19.5 per cent in metropolitan seats.
ABC election analyst Casey Briggs said Labor had been re-elected.
"We're confident of that," he said.
What wasn't clear if was despite a statewide swing of 20.1 per cent to One Nation was if they could win seats with a complex picture in the regions due to preference flows.
Liberal leader Ashton Hurn looked to retain her seat in the Barossa Valley with the assistance of Labor preferences but the party had suffered a 17.5 per cent swing against it with nearly five per cent of the vote counted.
Federal Liberal senator Anne Ruston said it was concerning to see the deterioration in her party's state vote.
"The people have spoken and there's been quite a large message sent," Senator Ruston told ABC TV.
Mrs Hurn had only had a very short time to lead the party to the election but the Bondi terror attack and the federal coalition's woes had "not done Ashton any favours at all", the senator said.
Earlier in the day, Premier Peter Malinauskas queued for almost an hour at a booth in his electorate of Croydon on Saturday morning, along with wife Annabel and children, Jack, George, Eliza and Sophie.
Mr Malinauskas said he had followed his election day ritual of going for a run before heading to the Woodville Gardens booth with his family.
"It's the first time I've voted with four kids, which brings its own challenges," he said.
The premier's four young children waited patiently in the poll queue, but were understandably wilting by the time their parents arrived at the ballot box.
They were rewarded soon after with democracy sausages, with their father telling them "team Mali, dad's got to go and do some work" as he rushed off for an appearance with Prime Minister Anthony Albanese.
Mrs Hurn voted at Angaston Town Hall in her Barossa Valley electorate of Schubert before heading to Adelaide to visit other booths.
One Nation leader Cory Bernardi was among the record 454,862 (34.5 per cent) South Australians who cast ballots at early voting centres up to a week ago.
Another 174,000 (13.2 per cent) requested postal ballots, meaning almost half the 1.3 million eligible voters had potentially voted before election day.
Adelaide University emeritus professor of politics Clem Macintyre said the rise and rise of One Nation had the potential to create a watershed in Australian politics and the end of two-party politics at federal level.
"If they do make a breakthrough, they're going to have to work hard to be a more serious and viable alternative government," he said.
"It's more frustration with the major parties … I think we can still say One Nation is a party of disaffected voters."
Flinders University public policy associate lecturer Josh Sunman said One Nation's discipline had been the surprise story of the campaign.
He said One Nation had delivered targeted messaging and candidate discipline.
"I was expecting a lot more candidate scandals and meltdowns," he added.
Hours after he commented, a UK court issued an arrest warrant for One Nation's Aoi Baxter over a charge of sexually touching a woman without consent, and he was swiftly disendorsed.
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