The Canberra couple will marry on Tuesday night, two years to the day since their civil union and little more than 24 hours before the High Court makes a finding in the federal government's challenge to the ACT's same-sex marriage law.

It will be an intimate ceremony for the pair, with just a few family members and friends.

Ms Watson said she would be disappointed if the High Court found in the federal government's favour on Thursday, but she was taking her chance while it was there.

''We're excited about the possibility,'' she said. ''We want the equality, the opportunity.''

They met on the dance floor of a Canberra nightclub four years ago.

They were friends for a while, ''but I knew I loved her'', Ms Watson said. ''I loved her as soon as I met her … she's my best friend. Even if it's only for 24 hours, we have to do this.

''It's such a positive step. Even if it is just for one day, it's a step in the right direction.''

Couples have a five-day window to marry before the High Court publishes a finding in the federal government's challenge to the ACT law.

Celebrant Jules Rattenbury said she had six ceremonies booked during that window and had been fielding more inquiries since Tuesday's hearing, at which the court reserved its judgment in the case. Ms Rattenbury said the same-sex couples knew there was a risk that they could find their marriage invalidated if the court overturned the ACT's law.

''They're well aware, but they realise that to make a change, people have to make a start,'' Ms Rattenbury said.

Jodie Vaile plans to marry her partner, Jen Simpson, but the couple have decided to wait until the High Court's finding is known, all the time hoping that the court will rule in the ACT's favour.

''We wanted to play wait and see,'' Ms Vaile said, rather than risk having a marriage annulled.

''If the High Court comes down in the ACT's favour, we will do so [marry] really soon.''

Ivan Hinton, the deputy director of Australian Marriage Equality, will marry Chris Teoh on Saturday at Old Parliament House. He said the small window for same-sex marriages in the ACT was important for couples to show their desire for change.

''When I was a kid and I was full of self-loathing, I was desperately seeking representation,'' he said.

''It may only be for five days, but if kids in Australia see these images and see that this is where we're heading, this is a good thing.''