Former rugby union star David Pocock is poised to become the first territory senator from a minor party.
Mr Pocock, a conservationist and former Wallabies captain, secured a strong vote in the federal election after his high-profile ACT Senate campaign.
He is now on the verge of unseating Liberal senator Zed Seselja, while Labor's finance spokeswoman Katy Gallagher is set to retain her seat.
However, the results remain unconfirmed, as Senate preference votes were not counted last night.
Before this election, Labor and the Coalition had held every ACT and Northern Territory Senate seat in history.
The 2022 ACT Senate vote may also make history by being the first in which a Labor candidate fails to be elected on first-preference votes.
In territory elections, candidates need a quota of one-third of all votes plus one vote — about 33.3 per cent of votes — to win a Senate seat.
As of late last night, Labor's combined vote was 33.2 per cent — its lowest ever tally, but almost certainly enough to secure Senator Gallagher's re-election.
The Liberal vote had fallen 9 percentage points to 23.4 per cent, while Mr Pocock has secured 22.1 per cent.
These results are provisional and will change marginally as the count progresses.
However, as most minor parties urged their supporters to back Mr Pocock over Senator Seselja, the prospect of a Liberal victory is highly unlikely.
Pocock pledges action on climate, integrity, poverty and women's rights
He soon had the support of more than 2,200 volunteers, rivalling the main parties, and was one of the most visible ACT candidates during the campaign.
In recent weeks, the prospect of his election sparked a strong response from both main parties, which had never previously come close to losing a Senate seat.
At a rowdy celebration in the city last night, Mr Pocock thanked his supporters but stopped short of claiming victory.
He said he would continue to press for greater integrity in federal politics, lower living costs for poorer Canberrans, and a safer community for women.
"And, after a lost decade of denial and delay, it's clear we have to get on with climate action and play our part," he added.
"We have to do what's good for our future and we have to do it in a way that everyday Australians benefit from."
Labor strengthens grip on all ACT lower-house electorates
In Belconnen, Labor supporters gathered to celebrate four local victories and the election of an Anthony Albanese-led government.
Labor retained all three of its lower-house seats in the ACT, securing favourable swings in each electorate.
David Smith retained the southern seat of Bean with 62.3 per cent of the two-party-preferred vote.
In the central electorate of Canberra, the Liberal vote collapsed 8 percentage points, leaving it lagging behind the Greens; Labor's Alicia Payne was returned with a 58.7 per cent win over the Greens' Tim Hollo on a two-party-preferred basis.
Meanwhile, Andrew Leigh won his fifth term, with a two-party-preferred result of 65.8 per cent in the northern seat of Fenner.
At the celebration last night, Dr Leigh said Labor had never taken its ACT seats for granted.
Dr Leigh added that the re-election of Senator Gallagher meant Australia would soon have its first finance minister from the ACT.
"Katy Gallagher will be thinking about the interests of the ACT as she sits down to put together a budget with [Labor shadow treasurer] Jim Chalmers," he said.
"That going to be enormously valuable for the ACT."
Meanwhile, in the bordering New South Wales electorate of Eden-Monaro, which includes the towns of Queanbeyan, Yass and Bega, Labor MP Kristy McBain was re-elected with a strong swing in her favour of 7.6 percentage points.
Seselja proud of 'positive policies' in face of 'well-funded' opposition
Senator Seselja on Sunday said the national results were disappointing for the Liberals, and he congratulated Labor on its victory.
But he pointed out that the precise outcome in the ACT remained unknown, with many votes yet to be counted.
"In the ACT, as we saw in other Liberal seats across the country, we faced an extremely well-funded and targeted Climate 200 campaign for the Senate seat," he wrote on social media.
"We ran on positive policies that would deliver tangible benefits to Canberrans, including the release of 2,000 blocks of land for new homes in the ACT, more support for first-home buyers, continuing to invest record amounts in Canberra and building in a strong record of economic growth and investment in national security."
Senator Seselja said outgoing Prime Minister Scott Morrison had steered Australia and its economy successfully through its "greatest peace-time crisis since the Great Depression".
He also thanked Liberal volunteers and, in particular, his family for their support.
"Campaigns are always particularly difficult for the families of candidates, and this election has been no different."