Britain's move to legalise gay marriage has sparked fresh calls for Australia to follow suit.
The breakthrough on Wednesday quickly reignited the gay marriage debate in Australia.
"If Catholic Spain can allow people to marry each other regardless of their gender, if Britain can do it, if the president of the United States of America thinks it is time to change, then Julia Gillard and Tony Abbott should back it as well," Mr Bandt said.
"This is a matter of not if, but when."
Last year both houses of the Australian parliament rejected a same sex marriage bill sponsored by Labor backbenchers. Mr Abbott refused to allow coalition MPs a conscience vote on the bill.
South Australian Environment Minister Ian Hunter, who married his gay partner in Spain last year, said the British vote should send a strong message to Australia's federal leaders.
All the countries we liked to compare ourselves to were adopting gay marriage and Australia was being left behind, he said.
"We are going to start to look like the deep south in terms of this issue," he told AAP.
Australian Marriage Equality national convener Rodney Croome said the majority of Australians would be pleased to see such resounding support for gay marriage from a country like Britain.
"But this will be tinged with embarrassment that Australia is falling further behind and may soon be the only developed, English-speaking country without marriage equality," he said.
Mr Croome predicted many Australian couples would travel to Britain, or even use UK consulates at home, to marry if the British bill became law.
"It is sad that these couples will not have their marriages recognised under the law of the country they live in," he said.