ACT Labor secretary Matt Byrne has made the shock decision to resign from his position, less than a year out from the next territory election.
Mr Byrne told The Canberra Times that he was leaving his role to follow his wife, who has been offered a diplomatic posting in Europe.
He wrote to ACT Labor president Angie Drake on Monday night to inform her of his decision. He will formally resign on November 29, and has offered to assist the party in finding his successor as quickly as possible.
Chief Minister Andrew Barr and Deputy Chief Minister Yvette Berry paid tribute to Mr Byrne, highlighting his pivotal role in steering Labor to victory at the 2016 election.
ACT Labor only last week unveiled its field of candidates to contest the 2020 poll, and Mr Byrne said he had been focused on preparing for the campaign before his wife's overseas posting was confirmed.
"My family comes first," Mr Byrne said.
"While it is a hard decision to make from a professional and political point of view, it was a pretty easy one to make from a family one."
Mr Byrne was appointed to the position of ACT Labor party secretary in 2014, after working for Ms Berry and Labor's national executive.
He said guiding Labor to victory in 2016 was a highlight of his five-year tenure, particularly given that "many pundits had written us [the party] off" in the lead up to polling day.
Mr Byrne said the party had introduced significant reforms under his watch, including giving members a greater say over preselections and reducing its reliance on donations from businesses.
He was also pleased the party had finally started to address "serious issues around bullying and harassment". Among his only regrets was that senior party figures did not act earlier to stamp out bad behavior.
The party's youth branch has been plagued by allegations of bullying and harassment in recent years.
Mr Byrne and Mr Barr last month admonished ACT Young Labor members after they were photographed posing with an Aldi bag. The picture was an apparent reference to a $100,000 donation allegedly made to NSW Labor, which is currently the subject of an anti-corruption commission inquiry.
"It is going to take time, but I am confident that [problems in ACT Young Labor] will be overcome," he said.
"We have been honest and upfront with people about our problems. We haven't tried to sweep these things under the carpet."
Mr Byrne said Labor would face a fight to win next year's election, but was confident it could secure victory and extend its hold on power into a third decade.
"It's an inevitable challenge when you have been in for so many terms," he said.
"But I think Labor is the only party in this town which can provide a platform and a set of policy ideas to show how we are going to govern for the next four years."
Ms Berry said her former staffer, and fellow member of Labor's left faction, had been a "diligent and respected leader".
"His insight and values driven approach has steered us to a number of strong election outcomes," she said.
"Under his leadership, the party has grown to a record number of members and Matt is leaving behind a stronger and more democratic ACT Branch. I will miss his counsel and good humour and wish him all the best for the future."