Australia's first same-sex marriages will take place just after midnight on Friday.

Two ceremonies are due to take place from 12.01am (AEDT) Saturday in Canberra to take advantage of a new ACT law.

The marriages will become the first legally-recognised same-sex unions in an Australian jurisdiction.

"This is something I've been working hard for for a very long time," Australian marriage equality spokesman Ivan Hinton said on Friday.

Hinton will marry his partner Chris Teoh later on Saturday.

The pair are among a group of more than 12 couples travelling from NSW, Queensland, Western Australia and Victoria to Canberra for ceremonies.

The ACT legislative assembly's marriage equality bill passed in October and comes into effect on Saturday.

It grants legal recognition in the territory to same-sex couples who chose to enter a dedicated union in a ceremony officiated by an authorised celebrant.

But the marriages may be short lived if a Commonwealth High Court challenge to validity of the ACT law is successful.

The Commonwealth argues the federal marriage act is the sole governing legislation.

The High Court has reserved its judgment on the matter until 12 December.

In recognition of the new law, Canberra's landmark Black Mountain Tower will be lit up in rainbow lights from midnight on Friday.