Labor and the coalition have seized on figures by the independent watchdog in a battle over big budget savings promises.
A Senate estimates hearing in Canberra on Monday was told federal government spending growth on public hospitals and schools will slow over the next decade.
This is despite the government arguing that it is not cutting health and education spending to the states and territories.
Parliamentary Budget Officer Phil Bowen told the hearing commonwealth spending on hospitals is expected to slow from a historical real growth of 3.6 per cent annually to 1.7 per cent over the period beyond the next four years to 2025/26.
The figure for schools would slow from 5.5 per cent annually to 2 per cent, following the decision made in the 2014/15 budget.
"This is proof the prime minister is lying when he denies he has cut $57 billion from hospital funding," Labor health spokeswoman Catherine King told AAP.
At the same hearing, coalition senators grilled Mr Bowen on Labor's policy of saving $14 billion over a decade by targeting wealthy superannuants.
Mr Bowen said every policy costing provided by the PBO to parties was given a ranking in terms of its "reliability", which ranged from low to highly reliable.
These assessments were confidential to the parties receiving them. But he acknowledged costings became less reliable if the policy extended beyond four years, as Labor's did.
"The further out you go, the less reliable the estimates - there is no doubt about that and we will always make this clear in the costings we prepare," Mr Bowen said.
Asked specifically about his costing of Labor's superannuation policy, Mr Bowen said he would be breaching confidentiality if he released the reliability rating.
Liberal senator Dean Smith asked Mr Bowen whether the Labor super policy costing "might not actually be true" because of its 10-year timeframe.
"I can acknowledge that our best estimate of the cost of this policy is $14 billion over a decade," Mr Bowen said.
Nationals senator Bridget McKenzie told AAP the costing of Labor's policy appeared unreliable.
"The community must have confidence in the information they are being presented," she said.
The PBO also revealed it would release a report this year into the distributional effects of the GST, to follow up on an OECD study released last December that left out Australia.
The PBO will also release its summary of the 2015/16 budget this week.
A study into spending trends affecting Medicare will be published by September.
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