Labor looks set to pick up the Adelaide seat of Boothby for the first time in more than 70 years, according to ABC election analyst Antony Green.
Boothby is South Australia's most marginal seat and was the focus of a predominantly three-cornered contest — between Labor, the Liberals and teal independent Jo Dyer — to replace outgoing Liberal MP Nicolle Flint.
With more than half the vote counted, the ABC's election computer is predicting Labor's Louise Miller-Frost has defeated Liberal candidate Rachel Swift.
Earlier, Green told ABC viewers that Boothby was among the "seats which are changing".
Tonight, Ms Miller-Frost all but claimed victory, taking to the stage flanked by her three triplet sons and husband.
"There are still some preferences to be counted, but I’m feeling really very hopeful."
Ms Miller-Frost told supporters the likely result reflected the views of the electorate on a range of issues.
"We went out there to talk to them about what they wanted," she said.
"They want action on climate change, they want a federal ICAC … they want action on aged care."
Going into the election, Boothby was South Australia's most knife-edge seat and was retained by the Liberals at the 2019 election on a margin of 1.4 per cent.
While its boundaries have changed over the decades, Boothby has been in Liberal hands since 1949.
Dr Swift earlier acknowledged the tense nature of the race.
"It is a close contest, we always knew it would be a close contest in Boothby," she said.
"I think it's going to be a long night. We still have a lot of pre-polls to count, a lot of postal votes to count, but the pre-poll so far has been really positive.
"I'll just continue to do what I've done for the past 12 months which is chat to the community and watch numbers come in."
On Twitter, independent challenger Jo Dyer acknowledged her own defeat, but said she had been proud of "what we added to the campaign".
"In theatre, there are no small parts — it's all about the impact you have when on stage," Ms Dyer tweeted.
"We had a big impact in the Boothby race."
Sturt too close, Stevens acknowledges need for 'rebuild'
The recent South Australian state election represented an electoral trouncing for the Liberal Party, and most of SA's 10 federal lower house seats have registered swings away from the Coalition.
While the seat of Sturt, in Adelaide's eastern suburbs, is not usually regarded as marginal, incumbent Liberal James Stevens has had a fight on his hands to retain it.
The current count has Mr Stevens marginally ahead of Labor's Sonja Baram.
Mr Stevens, who won the seat in 2019 on a 6.9 per cent margin after Christopher Pyne retired, said the party now had some serious challenges ahead as it confronted the prospect of a "rebuild".
"I hope that I'm re-elected here in Sturt. I believe that will be the case, but it is a tight contest and it's too early to call the result.
"We've got some challenges, there's no-doubt about it. There'll be another time to talk about those challenges and what we need to do to rebuild our great movement here in South Australia to be one that leads this state and leads this country forward."
Former SA deputy premier Vickie Chapman — who is at the Robin Hood Hotel, where supporters of Mr Stevens have gathered — was measured in her assessment, acknowledging Mr Stevens had a fight on his hands.
"It's obviously a difficult time for James but as you know, we've just experienced the state election again where there are multiple parties and significant independents," Ms Chapman said.
"We have a situation where it can be several weeks before we know the actual answer to who is going to be the local member."
Ms Chapman said the result would likely come down to pre-polls and postal votes, and said the number of candidates had had an impact on the outcome so far.
"Generally pre-polls will support a swing to the Liberal Party but we'll see," she said.
"We see a huge drop in the primary vote, splintering off across eleven candidates. I can't think of a time when we've had eleven candidates, it's the highest in all of South Australia.
"That just simply splinters the primary vote and naturally there's been this drift and how those cards fall as they drift off is really the key to this."
The regional seat of Grey has also recorded a swing against the Liberals, but it does not appear to be anywhere near enough to threaten long-time MP Rowan Ramsey.
Mr Ramsey's main challenger was independent candidate Liz Habermann, but he is currently ahead on a significant margin and the ABC is projecting he will retain the seat.
With more than a quarter of the vote counted, Centre Alliance MP Rebekha Sharkie looks to have secured the seat of Mayo, seeing off Liberal challenger Allison Bluck.
On the current count, Ms Sharkie has also picked up a significant swing towards her.
