The sister of Australian Prime Minister Tony Abbott has hit out at him for continuing to block same-sex marriage.

Mr Abbott, the leader of the Liberal Party, is a staunch opponent of same-sex marriage despite overwhelming support – and has consistently ducked calls to give his MPs a free vote on the issue.

After the Republic of Ireland voted overwhelmingly in favour of same-sex marriage, Mr Abbott was quick to rule out the possibility of a referendum happening in Australia, claiming referendums can only be held for constitutional changes.

Referring to a rejection of same-sex marriage in 2012, Mr Abbott added: “I don’t know if and when it’s going to come before our Parliament again. It came before our Parliament in the last term and was dealt with fairly decisively.”

However, his own sister Christine Forster – who is a lesbian – is not happy with her brother’s continued opposition on the issue.

Ms Forster proposed to her partner Virginia Edwards in 2013 – but is still waiting for her brother to catch up with public opinion and allow them to marry.

She said: “I’d like something more from the current prime minister… who happens to be my brother.

“I absolutely will be lobbying my brother, and I hope the party room makes enough noise about this.”

Mr Abbott has also admitted that most of his family is on his sister’s side in the split.

He said: “Inside the Abbott family I’m probably the last holdout for the traditional position, so look, it is a serious issue.”