The Prime Minister promised on Sunday night that a re-elected Labor government would hold a parliamentary vote on same-sex marriage within 100 days of being returned to office.

Mr Barr, who entered a legally binding civil union with partner Anthony Toms in 2009, praised Mr Rudd's support for marriage equality.

''Not only is it the right thing to do, I suspect that there will be a political outcome for him,'' Mr Barr said.

''A lot of people have been calling for this and it will appeal to younger and more progressive voters.

''It will play positively in the ACT seats and, on the national stage, I think there are at least half a dozen seats that are not held by the Labor Party where this will assist the Labor candidates.''

Labor has a conscience vote on same-sex marriage and Mr Barr said it was unlikely amendments to the Marriage Act would pass unless the Liberal Party allowed its MPs to do likewise.

''There would still need to be people from other parties voting in favour of marriage equality bills in order for them to be successful,'' Mr Barr said.

The ACT Legislative Assembly is due to debate, during its spring session, a bill to permit same-sex marriage in the territory. Mr Barr said it would not be necessary for the Assembly to debate the bill if the Federal Parliament legislated for same-sex marriage.

Renewed discussion in Australia on same-sex marriage came as it was announced that the new US ambassador to Australia, John Berry, has married his partner, Curtis Yee, in an episcopal church in Washington DC.

Labor Party members will rally in Civic at lunchtime on Tuesday in support of Mr Rudd's gay marriage push and federal ALP candidates.

Activists opposed to same-sex marriage have branded Tuesday ''National Marriage Day'' and will hold gatherings across the country.

National Marriage Day spokeswoman Mary-Louise Fowler said major events would be held in Melbourne, Sydney and Perth, in addition to smaller rallies in regional centres such as Wagga Wagga.

Ms Fowler said many voters were unhappy about the renewed attention on sex-same marriage legislation.

''Children need a mother and a father,'' she said. "To define marriage as anything other than a union between a man and a woman is an attempt to remove this fundamental biological link between children and their parents.''

Equal Love campaigners are also preparing for their own events, starting with the Rally for Marriage Equality in Melbourne on Saturday. The event, to be held at the State Library of Victoria, comes just four days after their opponents vacate the venue.

Equal Love spokeswoman Ivette Madrid said members would be hosting an additional rally in the capital on August 31, which political hopefuls, such as Greens Senate candidate Simon Sheikh, had been invited to.