The Greens and Tasmanian Jacqui Lambie could determine the future of Labor's parliamentary agenda, with the minor parties looking set to hold the balance of power in the Senate.
The Greens are on track to have their largest-ever presence in the upper house with 12 senators — two from each of the states.
If Labor does not have the Coalition's support, the Greens' support will be mandatory for any legislation it wants to pass. But the Greens alone will not be enough.
Senator Lambie looks to have gained a second Tasmanian seat, which will double her party's representation.
The ABC is projecting Labor will win 25 seats, short of the 39 needed for a majority in the Senate — something a government has not held since John Howard's prime ministership.
The Coalition will hold the most seats, likely 30, in the majority-female chamber.
If Labor holds 25 seats and gets the support of the Greens, Senator Lambie's two votes will be needed to pass legislation.
The future of Pauline Hanson, one of the nation's best-known politicians in the country, remains unclear.
Her party has one seat not up for election this term, with Senator Hanson in a fight to win the final Queensland Senate seat. Her candidate in South Australia could win the final seat in that state.
If both Senator Hanson and Jennifer Game win, One Nation will have three seats on the crossbench.
Former senator Nick Xenophon's political comeback has failed, polling behind One Nation and the United Australia Party.
Liberal Zed Seselja, based on ABC projections at 2:30pm on Sunday, is ahead of independent David Pocock. However, the former rugby star remains within striking distance thanks to preferences from Labor, the Greens and another independent.
The sixth seat in Victoria hangs in the balance, with Clive Palmer's United Australia Party ahead. That state looks to have re-elected two Labor senators, two from the Coalition and one from the Greens.
Greens leader Adam Bandt, who sits in the lower house, has dubbed the election a "Greenslide" and warned his party will use its balance of power to push for greater climate commitments.
"This result is a mandate for action on climate and inequality," he said on Sunday morning.
"We put an alternative to people and people liked what they saw. A record number of people voted Greens for the first time at this election."
Liberal frontbencher Anne Ruston was the manager of government business in the Senate and played a crucial role in crossbench negotiations during the last term of parliament.
Speaking on Saturday night, she said the Greens' wins could prompt Labor and the Coalition to negotiate more around policies.
"What it may actually deliver, though, is the fact that the two major parties may just have to start working a little more closely together to get sensible policy because I don't think Australians necessarily would like to see some of the policies of the Greens put into place by the Labor Party," she said.
"I think there is going to have to be a … new relationship between the two major parties to make sure we do get sensible policy, because we can get some pretty crazy policy if you're relying on the Greens to support Labor."
