New Greens MP Adam Bandt has nominated same-sex marriage and better treatment of asylum seekers as his first priorities when he enters Parliament.

Labor today formed an alliance with the Greens that means its MPs will have the ability to introduce private members' bills and have them debated and voted on.

Greens MPs would also be able to have their policies costed by a budgetary office of Parliament.

Mr Bandt says if Labor is able to form a minority government, he will introduce legislation on same-sex marriage and asylum seekers first.

"One is it's important to me to remove the discrimination against same-sex couples from marrying," he told ABC News 24.

"I don't think it has any place in a modern democracy, and I think every opinion poll says that people are in favour of allowing people to marry who they choose.

"Second thing that's important to me is ending mandatory detention and offshore processing of asylum seekers, and we've now got the capacity to move that through Parliament."

Under the Greens-Labor agreement, Mr Bandt will support a Gillard minority government in return for a range of concessions, such as the two-and-a-half hours of allocated debate for private members' bills.

With Mr Bandt's support, Labor has 73 seats alongside the Coalition's 73.

Opposition Leader Tony Abbott has accused Labor and the Greens of forming a coalition.

"What today's announcement means is that Australia now has two coalitions," he said.

"Only one of those coalitions will be good for regional Australia."

But Mr Bandt has denied that, saying the parties will differ on some issues.

"It's certainly not a coalition. There's certainly no agreement to vote together other than to ensure supply and to ensure confidence," he said.

"There's a range of issues where I think we are going to have a difference of opinion to the Labor Party.

"So for example asylum seekers, same-sex marriage - and we're going to continue to progress that as the Greens through the Parliament."