Speaking at an anti-offshore detention rally in Civic on Sunday, Mr Stanhope said if the current crop of parliamentarians would not change their minds it was time to change them.

A life member of the Australian Labor Party, and Australia's longest ever serving head of a federal, state or territory government, he told the 2500-strong crowd politicians who claimed to be "personally against" compulsory detention were hypocrites.

"You either support the policy or you don't, and the silence of Labor Party members in the face of the overwhelming evidence of the devastating impact of indefinite offshore detention can only be interpreted in one way; as support for the policy," he said.

Daniel Cotton, a member of the Canberra Refugee Action Committee steering committee, said Sunday's rally, which included a march by university students, academics, health workers, members of the LGBT community, firefighters, Rural Australians for Refugees, church groups and others, sent a powerful message.

"I wouldn't have imagined we would have this many people," he said.

"News of the latest legislation [being proposed by the federal government] may have influenced some but it is clear there is a solid base of opposition [to indefinite offshore detention].

"This is not a fringe or radical push; it isn't being perceived as such."

Mr Cotton said while CRAC would not call on people to join political parties, it recognised there were individuals within both major parties who were working to overturn the detention policy.

"We stand shoulder to shoulder with those people," he said.

Mr Stanhope took direct aim at his Labor federal parliamentary colleagues, saying: "There are ALP politicians anxious we understand they do not really support the policy. They say things like: 'it's part of our platform and I have to support it'."

He said it was time parliamentarians stopped turning away from the cruel and inhumane treatment of refugees.

"Those of us who are members of the ALP or the Liberal Party and who oppose these policies need to do more to ensure the candidates we preselect shall also not support these policies.

"If there ever was a reason to join a political party, there is currently none more pressing than in order to work from within the parties to have these unconscionable policies dropped."

One high-profile Canberran at the rally was Greens leader Shane Rattenbury, who went directly there from the press conference to announce his party's new deal with the Barr government.

"[Ending indefinite offshore detention] is a long-term issue that will only be resolved by Australians making it clear we want a better policy," he said. "Australia can do much better than this."

Mr Rattenbury said he attended the rally as a private individual joining other Canberrans in protesting bad laws.

"But if my public profile helps in anyway I am happy for that as well," he said.