Pledging to move quickly to into a new phase of governing, the 41-year-old economics minded former student politician, staffer and eight-year Assembly veteran will replace outgoing leader Katy Gallagher as she moves to the Senate in 2015.

Mr Barr said he would outline changes to the ministry and other governance arrangements in a speech after the vote, before a countback for the vacancy created by Ms Gallagher's resignation.

Nominating the deteriorating territory budget bottom line as an immediate and considerable challenge for the government, Mr Barr indicated he would likely hand responsibility for the Mr Fluffy loose-fill asbestos crisis to another minister in a full reshuffle due in January.

Health, education and housing will also be priorities, with the $800 million light rail line also expected to be front of mind for voters at the 2016 election. He will retain the Treasury portfolio.

"I am acutely aware of the challenges and the onerous nature of the role I am about to step into but I go into this optimistic about the future of our city, optimistic about the talented people that I will be working with and very much looking forward to the challenge of being elected chief minister," he said during a visit to Turner Primary School.

Mr Barr said Wednesday's caucus meeting featured mixed emotions amid tributes to Ms Gallagher who he called "a Labor hero" and role model for many Canberrans.

His current chief-of-staff Meegan Fitzharris is favoured by party strategists to win the required countback in the Molonglo electorate, but Mr Barr said she won't seek a position in the new ministry, clearing the way for Yvette Berry or another current backbencher.

"We've finally got there," he said in one interview after being asked about his historic position as the first openly gay head of government in Australia.

Incoming deputy chief minister Simon Corbell would not confirm if he will retain the Capital Metro ministry or again take on responsibility for the $1 billion asbestos clean-up.

Ministers have been told of their new positions in the ministry which will again include six front bench positions from next month.

"My task is to be both Andrew's rock, to be his sounding board and to be his most vocal supporter and advocate," Mr Corbell said.

Opposition Leader Jeremy Hanson immediately shifted focus from Ms Gallagher to her successor, saying only 4 per cent of voters had chosen Mr Barr as their representative and he lacked a mandate.

"No one at the last election wanted or thought Andrew Barr would be Chief Minister, and it will only likely occur… as a result of backroom factional deals and ALP backbenchers unwilling to stand up for their electorates."

"My door is open to talk to any ALP backbencher who has lost confidence in the flawed light rail plan," Mr Hanson said.

Mr Hanson will nominate for the position of Chief Minister in Thursday's sitting, but Greens minister Shane Rattenbury will vote with Labor to support Mr Barr.

Ms Gallagher will be recognised in formal speeches as part of the sitting. She is yet to name a date for her resignation from the Legislative Assembly, expected before Christmas.