The federal government is refusing to release more detailed modelling prepared before the budget by Treasury, that shows the likely impact of the proposed measures on different household types.
Documents released under freedom-of-information legislation to Fairfax Media show the government delivered its budget fully aware its spending cuts would hit poorer households much harder than wealthier ones.
May budget: Impact on low-income households was known. Photo: James Alcock
But two larger documents were withheld from the FOI request, one of 56 pages and the other of 21 pages. It is understood they show clearly how the less wealthy households would suffer far bigger falls in disposable income than richer ones, especially for families with children aged between six and 16.
A spokeswoman for Treasurer Joe Hockey said the documents would not be released because they were prepared for the cabinet and were therefore protected in the FOI process.
''Every government receives a range of advice and analysis during the budget preparation process,'' the spokeswoman said.
''[And] the Abbott government clearly shows what the welfare changes would mean, with 10 pages of material contained in the budget papers. This is significantly more than has been included in most previous budgets.''
Hockey criticised Fairfax Media's report of the Treasury analysis on Monday, saying the figures did not tell the complete story.
He also denied the data indicated the government knew its budget would hit the poor the hardest. He noted it ''fails to take into account the massive number of concessional payments such as discounted pharmaceuticals, discounted transport, discounted childcare that goes to lower-income households''.
However, all of these payments had been cut in the federal budget in one way or another.
Subsidies to pharmaceuticals have been reduced, federal funding for transport discounts are being withdrawn, and there are tougher conditions to get childcare benefits. All these changes will result in lower-income earners being worse off.
But shadow treasurer Chris Bowen slammed Mr Hockey on Monday for attacking the media for ''daring'' to report on the data, saying the Treasurer ought to be ''acknowledging [the] Treasury figures show the fundamental unfairness of the budget.''
''These are Treasury figures. These aren't Sydney Morning Herald's figures, not the Labor Party's figures - these are Treasury figures, which have been released under FOI,'' Mr Bowen told ABC Radio.
A former Treasury official, who did not wish to be named, said he had ''no reason to expect Treasury's figures are wrong''. ''And I have no reason to expect these figures will go away, either,'' he said.
Stephen Koukoulas from Market Economics said the government would have known that its budget was going to hit lower-income households hardest.
''The way that all these policies are actually costed and analysed, there is almost always an income distribution effect, particularly on these sorts of policies when its obvious there's going to be some sort of impact,'' he said.