Australian Prime Minister Julia Gillard inched closer to return to power on Thursday, after one of four independent lawmakers holding the balance of power threw his support behind her Labor Party.

Earlier on Wednesday, Greens' Member of Parliament (MP) Adam Bandt also signed a deal to declare support for Labor.

Tasmanian independent Member of Parliament (MP) Andrew Wilkie on Thursday signed an agreement with Gillard for his support of a Labor minority government.

In return, 100 million Australian dollars (91 million U.S. dollars) will be made available to building work at Hobart hospital where the independent member of Parliament (MP) Andrew Wilkie comes from.

Labor would also bring in mandatory pre-commitment technology on all poker machines as requested by Wilkie to combat problem gambling.

Independent Andrew Wilkie's decision to back Gillard means Labor can now claim 74 seats in the 150-member lower house of parliament. A total of at least 76 seats is needed for either party to form a government on their own.

The conservative opposition has 73 seats, but it could still win the race to form a government if the three remaining rural- based independents line up behind its leader, Tony Abbott.

However, the Treasury on Thursday revealed there is a hole of up to 11 billion dollars (10 billion U.S. dollars) in the Coalition's costings of its Australian general election promises.

The finding, as revealed to the three independent MPs on Wednesday, has forced at least one, New England MP Tony Windsor, to question his possible support for Coalition Leader Tony Abbott.

Windsor has urged the coalition to explain the discrepancy in estimated budget savings. While refusing to say if it will sway his final decision, Windsor said it has made him suspicious about Abbott and whether the coalition can be trusted.

"We probably understand now why he (Abbott) wasn't interested in releasing the numbers," Windsor told ABC Television.

"One of (the) things that we have to establish is trust in what they're actually saying."

Independent Oakeshott also wants an explanation from the Coalition and said the figures are concerning.

"That's a lot of money. It's more than what I've got in my wallet," Oakeshott said.

But the opposition was out in force on Thursday, arguing its figures still stood up to scrutiny. "It's a matter of difference of opinion, it's not a matter of error," opposition finance spokesman Andrew Robb said, arguing Treasury used different models and data in its assessment.

Meanwhile, Wilkie predicted that "one or two, maybe three" of the remaining independents would get behind the Labor.

News of holes in the coalition's policy costings could also advance this, Wilkie told a media conference in Canberra on Thursday.

"It's now more likely that the Australian Labor Party will get the numbers and perhaps even have a buffer to be able to provide stable government for the next three years," he said. The remaining three independents' decision may not come until early next week.