Tony Abbott has pledged to reveal some of the coalition's costings in coming days but will leave the final Budget bottom line until as late as possible before next week's election.
Under growing pressure to release the cuts necessary to deliver on his promise to bring the Budget back into surplus, the Opposition Leader said getting a handle on the fiscal pressures that he may face was becoming difficult.
Shadow treasurer Joe Hockey will debate Chris Bowen tomorrow with some of those costings expected to be released then.
Mr Abbott said the Opposition had already announced many detailed savings but more would eventually come.
"We won't be able to reveal the absolute final fiscal position until all of our policies are on the table, and that will be the final week," he said.
Mr Abbott denied he had backed away from a promise to deliver a surplus in his first term of government. "No, we will deliver a surplus as soon as we humanly can," he said.
"And we will deliver a surplus quicker and more reliably than the Labor Party.
"We are going to get back to surplus as quickly as we humanly can but we just don't know what the starting point is."
Treasury and Finance released their estimates on the state of the Budget a fortnight ago in their Pre-election Economic and Fiscal Outlook.
That forecast the Budget to reach a $4.2 billion surplus in 2016-17.
Analysis based on work by economist John Quiggin indicated yesterday the size of the cuts facing the coalition if it wants to dramatically reduce the size of government.
Over his first two Budgets, John Howard sliced spending by 1.1 per cent of GDP in 1996-97 and then 1.75 per cent the following year.
In today's dollars, that equates to Mr Abbott cutting spending by $17.1 billion in the next Budget and $27.5 billion in 2015-16.
Finance Minister Penny Wong seized on the figures, saying they were further proof Mr Abbott was deliberately hiding the Opposition's planned cuts.
"We know from history that the Liberals always cut too hard," she said.