Former Canberra Liberals leader Jeremy Hanson and a group of ACT opposition members have joined Coalition politicians supporting same-sex marriage, breaking from their conservative colleagues.
Mr Hanson, Nicole Lawder, Mark Parton and Elizabeth Lee joined Thursday's Canberra launch of the Libs and Nats for Yes group, Liberal and Nationals party members campaigning for a "yes" vote in the postal survey on same-sex marriage.
About 60 people attended the event in Civic.
The group, run by former acting federal director of the Liberal Party, Andrew Bragg, seeks to persuade voters of the case for same-sex marriage within conservative traditions.
Prime Minister Malcolm Turnbull helped launch the group last week, chaired by former NSW Liberal premier Nick Greiner and supported by federal government ministers Marise Payne, Kelly O'Dwyer, Simon Birmingham, George Brandis and Christopher Pyne.
Federal MPs also held an event at Parliament House on Thursday.
"The voices of the Liberal and National parties in this debate are some of the most important," Mr Hanson said.
"We represent middle Australia and they are the voters we will need to win the 'yes' vote."
Opposition to changing marriage laws has been a key difference between the Canberra Liberals and ACT Labor, and the presence of opposition members highlights the differences with conservative leader Alistair Coe and former leader Zed Seselja.
Senator Seselja is a key "no" advocate within the federal government and is closely aligned with Mr Coe.
Opposition members Vicki Dunne and Giulia Jones are among the party's most conservative voices. In August, Mrs Jones spoke at the right-wing Australian Family Association's national conference.
Former Liberal chief minister Kate Carnell and former senator Gary Humphries also support the "yes" campaign.
Mr Coe played down any differences on same-sex marriage on Thursday, saying the party reflected "a diversity of views in the Canberra community."
"The Canberra Liberals support the belief that all Canberrans, regardless of their views, should be treated with respect and should enjoy democratic freedoms to express ideas and voice opinions," he said.
"In much the same way, the Canberra Liberals support the belief that a truly robust and vigorous democracy is built on the back of many voices engaging in public debate."
Mr Coe said the party did not support the use of ACT taxpayers' money to support either side of the same-sex marriage debate.
The ACT government has spent $22,000 on two rainbow buses ahead of the postal vote.
On Friday, Chief Minister Andrew Barr will stage a media event in Civic during which he will post his "yes" response to the federal government's marriage survey.