Australia’s Prime Minister is keeping his options open on giving MPs a potential conscience vote on equal marriage – although Tony Abbott remains opposed to the reform.

Mr Abbott is supporting efforts to challenge the Austrian Capital Territory’s proposed equal marriage bill.

He said on Friday that the court challenge was about upholding the constitution and about preserving a “uniform approach throughout the commonwealth” to marriage laws.

Campaigners fear a legal challenge by the federal government could invalidate same-sex marriages performed after the bill comes into force later this year in the ACT.

Equal marriage remains banned at a federal level in Australia and efforts to legalise the measure failed in the Australian Parliament in September 2012.

But Mr Abbott suggests there is nothing to stop the Federal Parliament from reconsidering the issue.

With Labor and the Greens committed to marriage equality, and several prominent Liberal MPs in favour of a conscience vote, the government fully expects the issue to come back before the new Parliament.

However, it is unclear whether the absolute parliamentary numbers are there for a change in the law, even if Mr Abbott grants his MPs a conscience vote.