A result in the marginal South Australian electorate of Boothby may not be reached for several days, as first-term Liberal MP Nicolle Flint battles to retain her seat.
Ms Flint is running neck-and-neck with her Labor opponent Nadia Clancy after a swing of about two per cent against her.
She was on Sunday ahead 50.81 per cent to 49.19 per cent on a two-party preferred basis, with 75 per cent of votes counted.
The outcome will depend on pre-poll and postal votes, which are expected to favour Ms Flint.
Ms Clancy said it was too early to predict which way the seat would fall, but in any event the campaign had been hard fought.
"It looks like it will be a while until we know the result in Boothby but no matter the outcome I am so incredibly proud of what my team has achieved," she said on Sunday.
"We have fought a positive campaign focused on delivering a fair go for all South Australians, real action on climate change and investment in our schools and hospitals."
Either way, the result in Boothby was the only close contest across SA with results in the state's other nine electorates all as expected.
In the other seat of interest, Centre Alliance MP Rebekha Sharkie romped to another win in Mayo and looked set to slightly increase her majority after seeing off a second challenge from Liberal Georgina Downer.
Ms Sharkie said she would prioritise stronger policies on climate action, a federal Royal Commission on the River Murray, protections for the Great Australian Bight and a federal corruption watchdog.
"One of my first tasks will be to lobby the new government regarding all the promises made by the major parties in this election," she said.
"And I haven't forgotten my pledges to fight for an independent and adequately-resourced ABC, the phasing out of long-haul sheep exports and the creation of a River Murray research institute in SA."
In the SA Senate count, the Liberals looked likely to win three seats and Labor two, with sitting Greens Senator Sarah Hanson-Young favoured to take the final spot.
Australian Associated Press