The ACT Government made amendments to the Bill before it passed but the Liberal Opposition still has concerns the law contravenes the federal Marriage Act.

A spokeswoman for Mr Ruddock said the amendments "encouraged him" but the Attorney-General would not make a final decision until he had studied the amended bill.

"The Bill also still allows people as young as 16 to enter into a civil union. This remains a concern to the Liberal Opposition, as the age eligibility for marriage means one party must be 18 years old," Mr Stefaniak said.

"This is a very significant step forward in our push towards full legal equality under Australian law," Peter Furness, national convener for the lobby group Australian Marriage Equality, said.

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