ACT Greens leader Shane Rattenbury has written to federal Attorney-General Christian Porter to ask if the 31 Canberra couples who married in the 2013 wed again under the newly amended Commonwealth law, a retrospective date be recorded.

It would not give the couples automatic recognition of their previous marriage, but would be "the decent thing to do", Mr Rattenbury said.

"For those couples who were married in 2013, the day they gathered together with family and friends to declare their commitment to each other should be recognised as such," Mr Rattenbury said.

"These relationships have always been legitimate - and it's time our laws reflected this."

When same-sex marriage became legal across Australia last year, the ACT government offered to waive the $55 certificate fee for couples married previously under the ACT's laws.

The amended Commonwealth bill passed federal parliament on the fourth anniversary of the first weddings under ACT law.

Thirty-one couples wed in the week same-sex marriage was legal in Canberra in 2013.

But the marriages were voided after the High Court ruled the territory's parliament did not have the power to make legislation on marriage.

Mr Rattenbury first wrote to former Attorney-General George Brandis in December to ask that the couples' wedding date be recognised.

"In 2017, Australians chose love. In the postal survey, three in four Canberrans voted yes-leading the nation in our commitment to marriage equality," Mr Rattenbury said.

"But Canberrans have long supported marriage equality. We saw this in 2013, when the ACT Assembly passed local marriage equality laws, and 31 same-sex couples were married in the first week.

"Back then, we thought this the beginning of governments in Australia saying 'no' to the historical institutionalised discrimination that relegates same-sex couples to a second-class status.

"Sadly, the federal government opposed the change and challenged the legislation in the High Court, having the legislation and the marriages deemed invalid.

"Since then, the nation has come together to make marriage equality a reality [and ] all couples, regardless of their sexual orientation, deserve to have the day they were married recognised as such, under law."

A spokesman for Mr Porter said because the High Court found the 31 marriages were never legally valid, they could not be treated the same as foreign marriages, which are now recognised.

He said while it was disappointing for couples, they could now choose to legally marry under the Commonwealth's amended law.