The South Australian Liberals have conceded defeat in the seat of Black in Adelaide's south in a result that is a hammer blow to the party, which has now lost a second by-election in eight months.

Vincent Tarzia said voters in the seat had sent his party a message, with the count showing a significant swing towards Labor in the electorate which was formerly held by the man Mr Tarzia replaced as Liberal leader, David Speirs.

"But can I tell you while we've lost the battle for Black we're focused on winning the war in March 2026."

Well over a third of the vote has been counted and the results so far show major swings towards Labor and the Greens, with Labor's Alex Dighton significantly ahead of Liberal candidate Amanda Wilson on a two-party preferred basis.

Mr Tarzia said the Liberals had "time to turn the ship around" before the 2026 election, but said it was incumbent on his party to "review this result" and consider "the really clear message" from voters.

"They have sent us a message and we've got to listen to that message," he said.

"The Labor Party has run an effective, short, hard campaign and we've got to respect the message that has been sent to us tonight.

In her speech to supporters, Ms Wilson — who is the Mayor of Holdfast Bay — said she was "flattered and grateful" to have been able to run in the seat.

"Unfortunately, I wasn't successful tonight, but I go back to being the mayor," she said.

"I look at this foreshore in front of me and I'm so happy I still get to represent these people."

Tarzia could not 'have done more'

The by-election was called last month after the seat was vacated by Mr Speirs, who quit parliament after being charged by police with supplying a controlled substance.

"Those events have certainly caused their damage."

When asked what the outcome said about his own leadership, Mr Tarzia replied: "I don't think there's a great [deal] more I could have done."

Almost a third of the electorate had their say before polling day, with about 8,500 of the more than 26,000 eligible voters casting early ballots.

Counting of those votes will not begin until Monday — a fact that had led to predictions that the outcome would remain unclear for several days.

But within an hour of the count commencing, ABC chief elections analyst Antony Green had detected a "huge swing" towards Labor.

"Second polling place and a big swing is on," Green wrote on X, formerly known as Twitter, early on Saturday evening.

"A government hasn't taken a seat from the opposition at a by-election in a century and now we have two in a single term."

In March, Labor claimed victory in the by-election in Dunstan, a seat which, like Black, had also been in Liberal hands.

Black was created during a 2016 boundary redistribution and was easily won by Mr Speirs at the 2018 state election, but became marginal four years later.

While Mr Speirs retained the seat in 2022 by 2.7 per cent on a two-party preferred basis, there was a 6.5 per cent swing towards Mr Dighton.

Mr Dighton seems to have more than doubled that at the by-election, with counting so far showing a further swing of more than 13 per cent.

'Shoulders of giants'

At Labor's formal function, a jubilant Mr Dighton told supporters that he was "humbled and honoured by the trust" put in him by voters.

"I want to dedicate myself to keeping that trust, to working hard for this community, to making sure we're making a difference in the lives of everyone in this community," he said.

Mr Dighton, who has been deputy principal at Sacred Heart College in Adelaide's west, thanked his family, including his wife, children and parents.

While the Liberals had great expectations heading into the poll, they also had much more at stake.

A Liberal win would not have changed the numbers in state parliament, but the Labor win further erodes the state opposition's parliamentary presence, and compounds the damage done by the Dunstan result.

The Liberals have now been reduced, in a 47-seat chamber, to just 13 lower house MPs — two fewer than at the so-called "Rannslide", when Labor's then-premier Mike Rann was emphatically re-elected in 2006.

When asked earlier on Saturday about the political significance of a potential victory for his party, Premier Peter Malinauskas said he would "leave that to the commentators".

At the Labor gathering, Mr Malinauskas extolled the result as one that should be celebrated by all of Labor's rank and file.

"I've known Alex for a long time. We had the pleasure together a long time ago before he went on to bigger and better things, and to be such a committed educator," he said.

"None of this happens without a bigger family, without a bigger team, and of course I'm talking about the great South Australian Labor Party. Each and every one of you is magnificent, just really good people.

"We stand on the shoulders of giants."

Cost of living, education, environment weigh on voters' minds

At polling stations on Saturday, voters flagged the cost-of-living crisis, affordable healthcare, education and the environment as among the issues at the forefront of their minds.

"I've grown up in this area and now I'm raising my family in this area so I think it's really important to think about what the community needs," Labor voter Tamara said.

"Right now we need to look at the local school community, we need to look at the parks and areas around for the families."

Black is a beachside seat, and voter Alexei Photakis said concerns for the coastal environment had prompted him to back the Greens.

"I don't like the idea of any destruction to natural areas. I understand that when we build and construct, we do damage to the environment no matter what we're doing," he said.

"I'm very aware of erosion, and erosion near the coasts, so that's something that is definitely on my radar."

Adam Brown said that, among the considerations of most importance to him, cost of living was the "big one".

"I'd say cost of living and for me, I work in the healthcare industry, so the cost of healthcare," he said.

"Our natural parks here are just awesome, just such a good place to live because of those natural parks."