The ACT parliament's youngest member said the decision was a long time in the making and came after his progressive politics caused ructions within the right faction.

Mr Pettersson, who spoke with Mr Barr about his move on Saturday, said he was yet to make a decision on whether he would join the powerful left faction, but it was a "live option".

However, his defection comes after the CFMEU ordered its staff last week to join to the left faction, in a rumoured bid to shore up support for preselection candidate John Falzon.

It's also been seen by some as part of a broader move to wrest control of the left from the dominant CPSU, however sources close to the union denied that was the case.

It comes just a week before the party's conference.

The move fundamentally alters the balance of power in the ACT parliament. If Mr Pettersson does formalise his links to the left, Yvette Berry, rather than Megan Fitzharris, would be the next Labor leader, should Mr Barr leave.

There are five members of the left in the Assembly - Ms Berry, Rachel Stephen-Smith, Mick Gentleman, Bec Cody and Suzanne Orr. Mr Pettersson would make six.

The right also have five – Mr Barr, Ms Fitzharris, Joy Burch, Tara Cheyne and Chris Steel.

The 12th Labor member - Gordon Ramsay - is unaligned, although he used to be a member of the left and often sides with it.

While the shift leaves Mr Barr in the minority, Mr Pettersson said he did not expect any move against Mr Barr and would "not be part of" any such move, with Mr Barr "doing a wonderful job". Nor did he believe Mr Barr would quit parliament before the next election.

But if Mr Barr does leaves politics, deputy chief minister Yvette Berry would now almost certainly be his successor, following in the footsteps of her father, Wayne Berry, who led the party briefly in the late 1990s.

Mr Pettersson worked for the CFMEU before his election in 2016 and the union backed him for preselection, despite his long-standing membership of the right. He will need to join the left to get its backing at the next election.

Mr Pettersson's ex-staffer Michael Hiscox, who is now the CFMEU's executive officer, has also jumped ship despite being the junior deputy convener of the right faction.

It's understood new CFMEU secretary Jason O'Mara wanted union staff to be on the same side, as the recent preselection battle deepened the rift between the left and right factions.

That triggered Mr Pettersson to consider his position in the right.

"For a long time my politics have expressed themselves further to the left - and that was evidenced in preselection," he said, a reference to the CFMEU's decision to support him at the last election.

"I'm quite a progressive individual ... This is just formalising that."

The left faction and CFMEU delegates have endorsed Dr Falzon for preselection.

The right has not endorsed any candidates, although two members of the faction - lobbyist Kel Watt and Andrew Barr staffer Jacob Ingram - are running.

Members are expected to vote with their factions, although those in the right faction have more discretion than members of the left, who are required to "show and tell" their votes.

However, Mr Pettersson does not live in Dr Falzon's electorate and is therefore ineligible to vote for him for preselection.

But there are fears up to a third of Labor's right faction could also quit, which would severely weaken the faction ahead of ACT Labor's annual conference this weekend.

While around four members - including Mr Pettersson and Mr Hiscox - have defected so far, the right faction of ACT Young Labor is under pressure to make the leap.

Mr Pettersson is a former president of ACT Young Labor and is believed to have some sway within the wing.

It's understood the Electrical Trades Union is also defecting from the right to the left and will command some small block of votes at the conference.

It's understood two of the CFMEU's part-timers are yet to decide whether they'll choose their faction or the union.

The move has also been interpreted by some as an attempt to shore up the left before conference, where the party secretary is elected. Current secretary Matt Byrne, a member of the left, has been criticised for his handling of the dirtsheet circulated about Mr Watt and faces anger from the right. Mr Watt has called on Mr Byrne to resign or be sacked at conference. The boosted numbers ensures the left keeps the key post either in Mr Byrne or a left replacement.

But Mr Pettersson said the defections had nothing to do with the imminent party conference. Mr Byrne was safe in his job, and the left already had the majority of votes at conference even without the extra votes the defections would bring, Mr Pettersson said.

"There's no shenanigans about conference," he said, insisting this was simply about dealing with the "awkwardness" of him being in the wrong faction.

The defection comes in the context of more factional wrangling for preselection for the seat of Canberra.

The left has moved to lock in an interpretation of Labor's affirmative action rules that would guarantee Dr Falzon preselection for the seat of Canberra if Katy Gallagher got the backing of the party to return to the Senate, since women would already hold two of the four seats.