© Giulio Saggin/ABC News The Australian newspaper publishes a full page list of companies supporting same-sex marriage.

Some of Australia's biggest businesses have thrown their weight behind the push for gay marriage, with a full-page newspaper advertisement today.



Corporations including Google, Qantas, Optus and the ANZ and Commonwealth banks have put their names to a list of Australian businesses backing marriage equality.



Others supporters include law firms Slater and Gordon and Gilbert and Tobin, and the Football Federation of Australia.



The ad was published after Prime Minister Tony Abbott's sister Christine Forster told the ABC she believed gay marriage would be legal in Australia by the end of the year.



Ms Forster, who is engaged to a woman told Lateline she thought her brother knew the change was inevitable.



But she said she did not think her brother's personal opposition to gay marriage had changed.



"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," she said.



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.

