Same-sex couples in which one partner has British citizenship will be able to marry from June 27 in British consulates in Sydney and Perth, with Melbourne, Brisbane and Canberra to follow soon after.

In a move that will increase pressure on the Abbott government to allow a conscience vote on marriage reform, hundreds of Australian couples are expected to marry and can begin the process to book ceremonies in Sydney or Perth from Thursday.

The Abbott government signalled in March that it had no objection to British consulates in Australia being used to host same-sex weddings after same-sex marriage was legalised in England and Wales.

The move was hailed by marriage equality advocates as a major step towards marriage reform in Australia just four months after a High Court finding struck down Australia's first same-sex marriage laws in the ACT.

Couples who marry under the British laws will not have their marriages recognised in Australia.

Recognition of overseas same-sex marriages in Australian law is currently the subject of a parliamentary inquiry.

Australian Marriage Equality national director Rodney Croome said nearly 300 Australian couples had married in New Zealand since its government passed marriage equality laws last year and similar numbers would use the British laws to marry in consulates in Australian cities.

He said Australia was playing catch-up with other countries on marriage reform and he expected the weddings in June to renew the political debate in Australia.

"In particular it will increase pressure on Tony Abbott to allow the Coalition party room a free vote on the issue, in line with basic Liberal principles," Mr Croome said.