Former ACT Chief Minister Katy Gallagher will formally resign from the Legislative Assembly on Tuesday, bringing to an end her 13 year tenure as she prepares to move to the Senate.
Ms Gallagher wrote to Assembly speaker Vicki Dunne via email on Thursday to advise she would present a formal letter of resignation next week.
The move comes after the Labor Party's administrative committee endorsed her to take over the Senate seat being vacated by the outgoing Kate Lundy in 2015.
Ms Gallagher was endorsed after she was the only Labor nomination for the vacancy due in early 2015.
"I would like to thank the ACT Labor membership for giving me the honour of representing the ALP in the Senate," Ms Gallagher said.
"It's a huge responsibility and one which I will undertake to the best of my ability. My priorities will be to defend Canberra, to represent our local community and to work hard to see the election of a Shorten Labor Government in 2016.
The popular leader has been working from home in recent days, bringing a quiet end to her more than three years as Chief Minister.
Once the formal letter of resignation to Mrs Dunne is received, the Assembly Speaker will inform ACT Electoral Commissioner Phillip Green, who will prepare to conduct a countback of 2012 election ballots to fill the Assembly vacancy.
Mr Green will advertise the countback in the seat of Molonglo, giving candidates who were not elected 10 days to nominate before the preferences are counted and the winner declared.
The advertising period is expected to begin on January 5 at the earliest.
Labor strategists are confident Chief Minister Andrew Barr's chief of staff Meegan Fitzharris will win the countback
A spokesman said Ms Fitzharris has been on leave from the Assembly in recent days, with Jamie Driscoll taking over as the head of Mr Barr's newly formed team.
Mr Barr moved into the Chief Minister's office this week.
A new minister will be elected from the Labor caucus in January, but Ms Fitzharris has told Mr Barr she intends to begin her term as a backbench member.