The department of finance had just a week to assess the initial costings of the Rudd government’s schools building program, a Senate hearing has been told.

The cost of the Building the Education Revolution program originally was estimated at $14 billion, but blew out by $1.7 billion within a matter of months.

The program was part of the government’s $42 billion economic stimulus aimed at cushioning Australia from the impact of the global financial crisis and recession.

Finance head David Tune said that in January 2009 his department was given a week to cost the measure.

The department advised the government there would be a 90 per cent take-up by schools for the program.

“It was an estimate,” Mr Tune told a budget estimates hearing in Canberra on Wednesday.

“Circumstances can change, as they did in this situation and further money was required.”

Based on the information the department had at the time, it thought it was a reasonable “estimating assumption”.

But Liberal senator Scott Ryan queried that description, saying a Senate inquiry into the stimulus had been told there wasn’t any doubt about the estimates.

Mr Tune said the estimate was done at a point in time on certain assumptions.

“I think they were reasonable assumptions at the time, recognising that things might change,” he said.

Mr Tune was not secretary of the Department of Finance and Deregulation at that time.