South Australia will have two Greens senators for the first time since 2016, while Nick Xenophon and Rex Patrick have almost certainly failed to get elected.
One Nation has received the highest vote of any party after Labor, the Liberals and the Greens, but its lead candidate is not hopeful of becoming South Australia's sixth representative in the upper house.
With 592,000 South Australian Senate votes counted as of 4pm ACST, the Liberal Party has won two quotas, the Labor the same and the Greens just under one.
One Nation received 4.1 per cent of the vote, ahead of the United Australia Party on 3.3 per cent.
The ungrouped candidates, which included former senators Nick Xenophon, Stirling Griff and Bob Day, also received 3.3 per cent of the vote, while Senator Patrick received 2.1 per cent.
Incoming Greens senator Barbara Pocock said the next generation of voters was backing her party's policies.
"And it's not just about the climate change future, it's about how will I afford a house, how will I get an education?"
Senator Patrick told the ABC his chances of being re-elected were "low but not impossible".
Mr Xenophon — a former colleague of Senator Patrick in the Nick Xenophon Team — said he was waiting to see how the below-the-line votes fell in the Senate.
"I am not politically dead but rather on life support in an induced coma," he said.
Senator Griff has also lost his seat, after running under Mr Xenophon in Group O on the Senate ballot.
One Nation lead Senate candidate Jennifer Game was hopeful of winning a spot on the red leather after her daughter, Sarah Game, became South Australia's first state MP last month.
The older Ms Game said the large number of small parties with similar policies appeared to have split the right-wing vote.
"While I think it's mathematically possible, I don't think it's likely," she said of her chances of winning.
"I'm just really thrilled to have got this far, to be honest, so it'd be a very big surprise to me to be elected.
"It all depends how the preferences will flow."
Simon Birmingham and Andrew McLachlan were re-elected as Liberal senators, while Penny Wong and Don Farrell will spend another six years in the Senate for Labor.
