The Greens had previously feared the laws would undercut national efforts for marriage equality, but now say they will vote in favour of the changes after consultation with Canberra's gay and lesbian community.

But the party said it would look to pass a marriage act for same-sex couples that applied to the ACT, in the event that changes to federal marriage laws were blocked.

Attorney-General Simon Corbell's bill, which will be passed by the Assembly on Tuesday, will restore the role of ceremonies and celebrants in civil unions and give same-sex couples the same rights as people married under the Marriage Act.

The legislation will effectively reinstate the ACT government's original civil union laws, which were overturned in 2006 by the Liberal federal government, who said they were too similar to marriage.

Greens MLA Shane Rattenbury said the decision to support the civil union laws was not a change of heart by the party, which has previously said the timing of the bill could distract from the marriage equality debate.

Mr Rattenbury said the party had spent the past two weeks meeting with community organisations for Canberra's gay, lesbian, bisexual and transgender community to hear their thoughts on the bill.

''The message that has come through from the community is that there are improvements to civil unions legislation in this bill and on that basis we will support it,'' he said.

''That said, we remain committed to marriage equality and we will continue to campaign for that, including if the federal government doesn't pass it.

''We would follow Tasmania and introduce legislation for marriage equality in the ACT.''

Today marks eight years since the Howard government amended the Commonwealth Marriage Act to exclude same-sex couples.

Canberra residents will speak in support of gay marriage at a rally in Civic Square today organised by Equal Love Canberra.

Ainslie resident Ivan Hinton, who is one of the faces of an Australian Marriage Equality advertising campaign, said the civil unions bill did not ''replace the desperate need that our community has for marriage equality.''

The vice-president of the ACT branch of Parents, Families and Friends of Lesbians and Gays has been married for four years under Canadian law to his partner, Chris Teoh.

''I want to be able to call my partner my husband,'' Mr Hinton said.

''I want to be able to say that I am married - without an asterisk.

''If I was hoping to recognise the significance of my lifelong commitment to Chris - even with the Civil Unions Bill passed - I would still feel compelled to travel overseas because the relationship I have is a marriage.''

Mr Hinton said it was ''saddening'' that Canberra was moving towards civil unions at a time when countries around the world were moving towards marriage equality.

But he acknowledged the government's bill was attempting to ''provide Canberra residents with some more structure''.

Mr Corbell welcomed the Greens' decision to back the bill and said he was surprised by the party's initial hesitation.

''The ACT Labor government has a proud record of working to further the rights of same-sex couples, and this legislation is an important step forward,'' he said. ''… I look forward to seeing this civil unions legislation pass into law.''