The Australian Greens are waiting to hear Treasury Secretary Martin Parkinson explain why the national debt ceiling should be raised to $500 billion.
However, lower house MP Adam Bandt has stopped short of saying how Mr Parkinson's evidence to a Senate estimates hearing may influence the Greens' position on the federal government plan.
"We are open to taking expert advice on matters central to the Australian economy," he told reporters in Canberra on Monday.
"But we'll wait and see what Martin Parkinson has to say, because it is an extraordinarily large amount of money, and we want to know why (it is necessary)."
Mr Parkinson will be grilled later this week on the plan to increase the debt cap from $300 billion.
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Labor and the Greens have so far resisted the government's attempts in parliament to legislate the rise, with Labor arguing it shouldn't be raised higher than $400 billion.
"We want to have the case made to us," Mr Bandt said.
"At the moment the government's approach seems to be to thumb its nose at the parliament."
Opposition Leader Bill Shorten says Treasurer Joe Hockey hasn't made the case for the increase.
"If the Treasurer of Australia can't make the case to the Australian people, why should they give him a higher debt cap?" he asked.
"I've noticed a trend with the Abbott government. They were a government in waiting, now they are a government in hiding."
Labor continues to demand the government release the mid-year budget update to reveal the true state of the books.
Mr Hockey has repeatedly said he won't release it until mid-December.
The current $300 billion debt cap is due to be reached on December 12.
