Mr Barr was given a hero's welcome at the Labor Party's official function in Belconnen, where a jubilant crowd chanted "four more years" and "build the tram" as their leader claimed victory in a speech just after 10pm.

"I can confirm that Shane (Rattenbury, Greens leader) and I will work together to form a new progressive government in the ACT. We don't yet know the final makeup of the Assembly, and these negotiations will continue into the next week," Mr Barr said.

"But I can confirm, having spoken with Shane, there is absolutely no doubt that we will form a government in the coming week."

The swing towards Labor, which even many of those within the party had not predicted, made for an elated and buoyant mood at the party's official function at the Belconnen Labor Club.

Mr Barr credited the result to a strong, organised campaign, and the most extensive effort in the field that Labor had undertaken.

He described the vote as a complete rejection of conservatism and the Liberals' narrow view of the city, speaking of Labor's commitments to renewable energy, light rail and a positive vision for Canberra's future.

"The strong progressive vote that both our parties [Labor and the Greens] have received, and the massive swing against the Liberals – it was a fundamental rejection of a narrow-minded, conservative agenda," Mr Barr said.

He said he had spoken with his opponent, Liberal leader Jeremy Hanson, and thanked him for a campaign that focused on policy, rather than personal attacks.

Labor were predicted an outside chance to win 13 seats, and had a swing toward them of 0.2 per cent by 10.30pm.

That was contrary to the views of many pundits, who believed Labor would suffer some form of protest vote over a range of issues, including light rail, and perceptions of issues of integrity.

Labor's Yvette Berry, generally regarded as the most likely deputy, said her team's work in the field had given them confidence of a strong result.

"We had hundreds of thousands of conversations with people and it was clear that what we were hearing in the media or from other sources wasn't what was happening on the ground," Ms Berry said.

"Canberrans love their city, and all they want to do is make it better. That's what Labor's been doing."

Labor minister Mick Gentleman also spoke of the strong ground campaign and its influence on the result.

Labor held an exhausting 302,000 conversations with voters this year, through doorknocking and phone calls. It attempted 919,000 conversations with voters.

"In my electorate we've been working really hard. My team has done 30,000 direct voter contacts, and that's reflected in some of the early results we've seen," Mr Gentleman said.

"We've also seen a rejection of the Liberals' negative campaigning down there."

ACT Labor secretary Matthew Byrne spoke at the event earlier on Saturday night.

He thanked volunteers and candidates for the massive field campaign over many months.

"It was the most significant community campaign that this party, in fact any party in the ACT, has ever seen," Mr Byrne said.