Greens leader Shane Rattenbury would be in the running for Attorney General if Labor's Gordon Ramsay loses his seat, Mr Barr said.

Mr Rattenbury will be given a senior ministerial role and, depending on the final number of Greens members, one of his colleagues could be given a more low-profile portfolio.

It's a softening of Mr Barr's previous comments that a first time MLA may not be ready to become a minister.

Labor and the Greens on Thursday began formal negotiations to hash out a new parliamentary agreement.

"Discussions are being undertaken in very good spirits," Mr Barr said.

Mr Rattenbury said the party expected to have at least two ministers in cabinet.

He said history didn't dictate that a first-term MLA should not be made a minister.

"Both Rachel Stephen-Smith and Gordon Ramsay were made ministers right at the start of last term, immediately upon being elected," he said.

"If we delve back into the previous decade, the Chief Minister became a minister almost as soon as he became a member of the assembly.

"We've got some really talented members, we've got a real depth of capability and I think we've got members capable of stepping up to that."

He remained tight-lipped about what portfolio he wanted to take on, and whether he would consider the attorney-general role.

But he said the party wanted to have the biggest say around climate change, planning reform, and housing.

Mr Rattenbury said the parliamentary agreement between the two parties would need to be comprehensive across a number of policy areas.

"Both Labor and Greens brought quite a lot of polices forward in the election campaign, some of those are quite similar but there's other areas where we have taken quite different approaches," he said.

"We won't be able to do everything both parties wanted to bring to the election."

Mr Barr said the final number of Greens ministers would depend on how the final two seats landed.

"As way of explanation to my comments [about not appointing a first-term minister], I'm not going to dump someone in the deep end where they don't know anything about a portfolio," Mr Barr said.

"But I think there are certainly good, qualified candidates within the Greens party for certain portfolios."

Mr Barr said there was also the option of adding a ministerial position when the members had settled in.

"I want to have those discussions and see the CVs and claims, I don't know who the Greens are going to put forward," he said.

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While there will need to be a reshuffle, he said he would not consider moving some ministers, including Health Minister Rachel Stephen-Smith.

Mr Barr said gender diversity and a spread of ministers across geographical areas would also be considered in the final makeup of his cabinet.

The Greens will have between five and six representatives, up from two in the last term, while Labor will have between 10 and 12.

There are still two seats yet to be decided, including Mr Ramsay's seat of Ginninderra.

He has fallen behind the Liberals' Peter Cain in counting.

The Greens and Labor are also locked in a battle for the fifth spot in the southern seat of Brindabella, with just a handful of votes separating the parties.

Mr Barr said he was sweating on Mr Ramsay's seat in Ginninderra, but wasn't ready to write him off yet.

"I am riding the roller coaster with my candidates," he said.

"Gordon Ramsay is a very significant part of our team , I'd be very upset if he doesn't get over the line. It's a very important outcome for us."