By Jacob Shteyman in Adelaide

Cressida O’Hanlon has a nervous wait ahead of her as she hopes to be confirmed as the next member of South Australia’s House of Assembly.

The 51-year-old business mediator was on track to win the seat of Dunstan for Labor on Saturday night – a historic victory if pre-poll votes continue to fall her way on Monday.

Not in more than a century has a government managed to win a seat off the opposition at a by-election, but with a 2.9 per cent, two-party preferred lead over her Liberal rival Anna Finizio, Ms O’Hanlon looks set to do just that.

“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.

With almost half the vote counted, ABC election analyst Antony Green said she was on track to claim the seat on the back of a 3.4 per cent swing.

Dunstan became the state’s most marginal seat after former 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 after Mr Marshall resigned in January following 14 years as member for the inner-eastern Adelaide electorate.

More than 8000 early votes out of an expected 27,000 will not be counted until Monday.

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

A government has not won a seat off an opposition in a by-election in South Australia in more than a century.

Regardless of the final result, Premier Peter Malinauskas will be buoyed that his party was in the contest at all.

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 the premier as he enters the second half of his first term in power.