The party's left caucus in the ACT endorsed Ms Gallagher late last week as its preselection candidate as she attempts a return to the upper house after her exit in May when a High Court decision found her ineligible to sit in parliament.

Ms Gallagher said she was taking nothing for granted entering the next stage of ACT Labor's Senate preselection contest, which is expected to end with a ballot of members in August determining the party's next federal election upper house ticket.

"I will work hard to win the support of ACT Labor branch members to run as their candidate for the Senate ticket and return another progressive Labor voice to the Senate," she said.

Her replacement in the Senate and right faction member David Smith did not comment when asked about the left caucus endorsement for Ms Gallagher.

He has previously said he would not stand aside, and would also vie for preselection, setting the scene for a showdown between the former running mates and their factional allies.

Senator Smith, a former right faction convenor, was second on the ticket when Ms Gallagher was elected to the Senate in 2016.

Ms Gallagher, who is from the left faction, first entered the Senate to fill a casual vacancy in 2015, but the High Court found she was still a British citizen when she nominated for the 2016 federal poll, rendering her ineligible to serve in parliament.

Her Senate re-election bid has received backing from Labor senator Penny Wong, who told the ABC's Insiders program earlier in June that senior figures within the party, including leader Bill Shorten, wanted Ms Gallagher to return to the upper house.

But Senator Smith said then there was "nothing on the public record" to indicate Mr Shorten did not want him in the Senate.

ACT Labor's administrative committee in July will set the preselection ballot date.