Pro-marriage equality MPs and constitutional law experts have warned the ACT government its same-sex marriage laws could be struck down in the High Court if the bill is not amended.
Interstate MPs, including the NSW independent MP Alex Greenwich, have called on the government to reword the bill, after presenting legal advice that it could be invalid because it does not create a separate status of marriage for same-sex marriage.
Two legal opinions, including one by constitutional lawyer and academic George Williams, argue the wording of the bill should be changed to strengthen its chances of surviving the Abbott government's High Court challenge.
The ACT Attorney-General Simon Corbell has refused to act on the advice, but talks were continuing on Sunday in the hope the bill might be amended before Tuesday's debate in the ACT Legislative Assembly.
Mr Corbell said on Sunday that "any suggestions that without amendments our bill is fatally flawed are wrong".
The legal advice, seen by Fairfax Media, says the ACT bill is at risk because, unlike bills in other states, it does not create a new type of marriage called same-sex marriage.
It argues the bill is problematic because it seeks to allow marriage for people who cannot be married under the federal Marriage Act.
Same-sex marriage bills to be debated in Tasmania and New South Wales have tried to minimise the risk of inconsistency with the federal act by creating an entirely separate form of marriage for same-sex couples.
In separate advice to Mr Corbell, Professor Williams also urged amendments to the bill, which "has been drafted in a way that exposes it to an additional likelihood of being struck down in the High Court."
Professor Williams said he was concerned the ACT government had not maximised the bill's chances of surviving the Abbott-government's High Court challenge and that a defeat in the High Court could disrupt the momentum for change in the states.
"You can't reach a conclusion it would be invalid," Professor Williams said on Sunday.
"It's reducing the risk is what it's about.
"I've indicated some things that could be done to reduce the risk."
Mr Corbell said on Sunday the government did not agree with the advice from interstate, partly because the test of consistency for the territory differed to the way consistency was tested for the states.
He said the test for the ACT was set out in section 28 of the self-government act, while for the states it was section 109 of the constitution.
Mr Corbell said a territory law "is valid to the extent it is capable of concurrent operation" with federal law. "These arguments are finely balanced," he said.
"The government has closely considered these issues and we have received these representations but on balance we see no need to amend the bill."
The Attorney-General said the bill would proceed as tabled, with one small amendment to do with the transition from civil unions into marriage.
Mr Greenwich, who is among a group of NSW MPs who will introduce that state's same-sex marriage bill, urged the ACT government to reconsider its position.
"Mr Corbell has been given expert legal advice on how to further strengthen the bill," he said. "No one can predict what the High Court will do but the safer option would be to follow in the footsteps of NSW and Tasmania.
"For the sake of the couples getting married I hope this is taken into consideration."
Advocacy group Australian Marriage Equality also has concerns about the bill and on Sunday said it hoped the government was doing everything it could to "Abbott-proof" the laws.
"We remain hopeful that the ACT government is operating on strong advice," deputy director Ivan Hinton said.
"We would like to ensure that this bill has the greatest opportunity to survive a High Court challenge."
The Legislative Assembly will debate its same-sex marriage bill on Tuesday and the bill could pass as early as that day.
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