Labor insiders are not expecting any candidates other than Katy Gallagher to contest Kate Lundy's Senate spot, with the left and right factions expected to unite behind Ms Gallagher's nomination.

The right is expected to support her because it delivers the faction the first right-faction chief minister in the territory's history, Andrew Barr.

ACT Labor will open nominations on Monday, December 8, for Senator Lundy's seat, giving party members 10 days to put up their hand to stand. If no one else seeks the job by December 18, Ms Gallagher will be declared as the party's nomination, which needs a formal process through the ACT Legislative Assembly before it goes forward to the Senate.

Senator Lundy will resign her Senate seat on March 31, enabling Ms Gallagher to take up the seat on April 1.

Who takes her seat in in the ACT Legislative Assembly will be decided by countback of ballot papers at the last election, with Meegan Fitzharris winning the most first preference votes by some distance and the frontrunner, but public servants Angie Drake and Mark Kulasingham and IT businessman David Matthews also in the running.

The result depends on where people who voted for Katy Gallagher directed their preferences, with the possibility that at least some preference flows will show a female vote.

The election of Ms Fitzharris would alter the factional balance in the ACT caucus, with the left no longer dominant. Ms Fitzharris, Mr Barr, Joy Burch and Mary Porter are in the right faction; Simon Corbell, Mick Gentleman and Yvette Berry in the left; and Chris Bourke is non-aligned.

Mr Barr will not only become the territory's first right-faction chief minister, but also, it is suggested, the first openly gay leader of a state or territory in Australia.

As Treasurer, he contends with a budget in tatters from the Mr Fluffy buyback, and as chief minister Canberrans can expect a tighter control on spending and determination to bring the budget under control, presumably mainly through revenue measures, with little discretion in spending, other than on projects such as light rail.

As leader of a government without the strength of Jon Stanhope or the popularity and level-headedness of Katy Gallagher, he also faces the challenge of taking Labor into the 2016 election.

Mr Barr is articulate and forceful, if sometimes impatient and dismissive, but the team he leads is less than compelling or experienced, other than in the old hands like the visionary Simon Corbell whose future in the Assembly is not clear.

If Mr Corbell takes the deputy's job, he is presumably committing to stay for the election, and beyond, to see through his big-picture projects - renewable energy and light rail, albeit in the face of tight hand on the budget.

Ms Gallagher's move is a happy occurrence for the Liberals' Jeremy Hanson, whose chances at government have just increased significantly, even if the team he leads looks no more convincing than Labor's.

If anyone else does nominate for Senator Lundy's seat in the 10 day period, a ballot will be called of all eligible party members. ACT Labor has about 1500 members, but to be eligible to vote in a pre-selection, a member needs to have attended a certain number of meetings in the past year (three meetings for people who have been members for between one and five years; two meetings between five and 10 years; and one meeting for people who have belonged 10 years or more). The ballot would close on January 31.

But no one is expected to challenge Ms Gallagher. Fluffy campaigner Brianna Heseltine, who had been tipped for the seat, is not eligible, having not been a member for two years, as required under the rules.