So far the tactic has worked for the Coalition. Without access to the documents and only to the numbers the Coalition says they have produced, Labor has been suckered in to making wild and incorrect claims about Coalition errors.

Journalists know the claims are wrong: they have been shown the budget office documents; they have been told they can make notes, but not copies, before Coalition staff whisk them from their hands.

When, operating in the dark, Labor claimed there were $10 billion in errors in the Coalition's costings, the Coalition calmly sent its costings to Treasury so it could see there were not.

A $5.2 billion saving from losing 12,000 public servants is a case in point. In April, Labor asked the office to cost a loss of 20,000 staff. The office said that 4000 staff were going in any case, and costed a cut of 16,000 - a saving of $4.26 billion.

Labor's Penny Wong thundered that the office had found "that even if Mr Abbott cut an additional 8000 jobs - taking the total to 20,000 - he would still be $700 million short of the savings he claims." It hadn't. It had costed a cut of 16,000.