With the Labor leadership ballot locked in for Monday, questions are turning to what might happen if Kevin Rudd wins back the prime ministership.

The former foreign minister has not confirmed he will stand against Prime Minister Julia Gillard, but will make a "definitive statement" on the issue later today.

If Mr Rudd does run and secures the support of the Labor caucus, there are still some constitutional hurdles he will have to overcome.

Constitutional law expert Anne Twomey says he would need to secure the support of Governor-General Quentin Bryce before he moves back into The Lodge.

"The first thing to know is that it doesn't automatically make him prime minister. In order to be prime minister the Governor-General needs to appoint him," she told ABC News Breakfast.

"First of all you need a vacancy, and that would mean Julia Gillard would have to resign. Assuming she did resign, then the Governor-General would have to make a choice.

"A lot of people don't realise that it's actually a reserve power of the Governor-General; she has some discretion in making this decision, but it's a discretion confined by some convention.

"The convention says she has to appoint the person who either holds the support of the majority of the Lower House or is most likely to hold that support."

Professor Twomey says that is where independent MPs will play a key role.

"Can she feel certain that the independents will support Kevin Rudd and that he is the one who holds the support of the majority of the Lower House?" she asked.

"It might be quite difficult for the Governor-General. There's two ways she could approach it.

"The first would be to ask the independents to come in or give a letter telling her which way they intend to vote - you saw that sort of experience more recently in Tasmania with the hung parliament there.

"Alternatively she could ... wait for parliament to sit, and let there be a vote of confidence or no confidence on the floor of the parliament and whatever the House of Representatives chooses then that person [will be appointed] as prime minister."

"And that would make sure she wasn't seen to be in any way biased or influencing results - she could leave it to the House to decide."

Governor General Quentin Bryce will play a key role if Kevin Rudd wins the leadership ballot ( Giulio Saggin: ABC News )

If the independents backed Opposition Leader Tony Abbott, he could arguably present a case to Ms Bryce to form government.

"It may well be with the independents that they don't support a Liberal government but they do support holding an election," Professor Twomey said.

"The independents can't themselves advise the Governor-General and say 'hey, call an election', but what they can do is say 'we will not give support to the new Labor leader, we will support Tony Abbott to the extent that when he comes into power, the first thing he's going to do is advise the Governor-General to call an election'.

"So that would be the way of causing an election if the independents thought that was the way to resolve the matter."

Professor Twomey says Ms Bryce also has the power to dissolve the parliament - but that is not a simple step.

"The problem with her power to dissolve parliament is that she does need to be advised by a prime minister to do it, so she can't do it off her own bat," she said.

"The way of getting around that is to appoint someone that will advise her to dissolve the parliament.

"So if the Governor-General is inclined to think this whole issue that should be resolved by an election, she would be able to appoint Tony Abbott who presumably would not want to be stuck dealing with independents and therefore would advise her to call an election - that would be the way to achieve it."