Labor elated, Greens pleased after SA by-election swing

Labor's Cressida O'Hanlon is nervously waiting, hoping to be confirmed as the next member of South Australia's House of Assembly.

The 51-year-old business mediator is on track to win the seat of Dunstan with a 2.9 per cent, two-party preferred lead over Liberal rival Anna Finizio.

No SA government in more than a century has managed to win a seat off the opposition at a by-election but Ms O'Hanlon looks set to achieve a historic victory should pre-poll votes continue to fall her way.

"Tonight we are in an incredible position and it's thanks to all of you," she told a room full of volunteers on Saturday night.

Attending the launch of a new medical complex on Sunday with Ms O'Hanlon at his side, Premier Peter Malinauskas declared she was "on the cusp of achieving something quite special".

The Greens emerged on Sunday as another potential success story, with Katie McCusker polling 22.4 per cent of first preference votes - an advance of some nine per cent on the party's return in the electorate at the 2022 state election.

"If we can replicate the results that we saw in Dunstan last night, we will win the seat of Sturt at the next federal election," SA Senator Robert Sims said on Sunday.

Federal Greens leader Adam Bandt declared Ms McCusker's showing had put the major parties on notice.

With almost half the vote counted, ABC election analyst Antony Green said Ms O'Hanlon was on track to claim the seat with a 3.4 per cent swing.

Dunstan became the state's most marginal seat after former Liberal premier Steven Marshall survived a 6.9 per cent swing against him to hold on by 0.5 per cent at the last election.

The by-election was forced when Mr Marshall resigned in January after 14 years as member for the inner-eastern Adelaide electorate.

More than 8000 early votes out of an expected 27,000 won't be counted until Monday.

If the final result confirms a Liberal loss, Opposition Leader David Speirs faces a huge task to reassure his party he is the right man to carry it to the next election.

Voters vented their frustrations in recent by-elections in Queensland and the federal seat of Dunkley with large swings against the government.

The apparent swing towards Labor serves as a huge fillip to Mr Malinauskas as he enters the second half of his first term in power.