Prime Minister Tony Abbott's sister Christine Forster says gay marriage will be legal in Australia by the end of the year and she thinks her brother knows it is inevitable.

Ms Forster, who is engaged to a woman, told Lateline it is wonderful that the change to marriage laws will come under a conservative, deeply religious Prime Minister.

"That will be for me one of the proudest moments of my life," she said.

"To think that this reform comes under the prime ministership of Tony Abbott ... I think it's entirely appropriate that it would come under the prime ministership of Tony Abbott.

"At the end of the day I'm sure he will understand that this is good and fair for everybody and it will be great for Australia."

The same-sex marriage debate has gained momentum this week on the back of Ireland's hugely successful referendum at the weekend.

Mr Abbott said on Wednesday the decision on whether or not Liberal MPs should have a free vote on the issue would be decided in a party room debate.

He also indicated there should be bi-partisan support for the issue.

Ms Forster, a Liberal Councillor for the City of Sydney, said the Irish referendum was a watershed moment.

But she does not think Mr Abbott's personal views on the issue have changed.

"The Irish decision, watching what unfolded in conservative Ireland ... to see that astounding result ... that would have told any thinking opponent of this reform that it is inevitable in a developed Western democracy like Australia," she said.

Ms Forster said she did not think Mr Abbott would campaign against the change.

"It's not up to Tony to allow or disallow anything. There will be a debate and there will be a decision made by the party room," she said.

"I'm sure the views of the senior politicians carry more weight than the others and I suppose the view of the leader carries the most weight ... but there will be a discussion and I'm sure there will be a range of views."

Ms Forster said she would marry her partner, Virginia, when the law is changed in Australia.

"I haven't checked in with her lately but last time we discussed it she was still game on," she said.

She said Mr Abbott would attend the wedding.