School principals in WA will have their normal last-day-of-school celebrations interrupted, as the Government has decided to give them their 2015 budgets just hours before the school year ends.

The WA Education Minister has had to defend the decision to hand out 2015 budgets on the final day of the 2014 semester.

The new outlines will leave a number of schools several hundreds of thousands of dollars worse off, and cause some big holiday headaches for administrators at schools statewide.

Some schools will find themselves almost $500,000 worse off across the five-year transition period.

Unions say it is a gutless move.

State School Teachers Union president Pat Byrne has told the ABC that principals have been lumped with outdated budgets and forced to make hasty changes.

“It is simply outrageous that principals are told on the last day of school that this is going to be the case,” she said.

“We've been told the Government has known about this for the past two weeks and have sat on it, basically.”

Education Minister Peter Collier told reporters it could not have been done any earlier.

“At the end of the year, that's when we have much more of a clearly identifiable figure in terms of each school,” he said.

“And then, of course, the final figures will come about with the census in mid-February which has been the case for decades upon decades.

“Principals were told unambiguously and have been told consistently since [July] that they were preliminary budgets and when we had the final figures with regard to student numbers and the make-up of those students, those figures would be refined.

“Then of course, as is the case every year, in February of next year when the final census is done that's when we will finally know.”

Opposition education spokesperson Sue Ellery said it proved the failing of the Government’s new procedures.

“It is astonishing to me that this new model is so flawed that the final figures could not have been prepared earlier,” she said.

“It is deceitful and disgraceful that the Premier has decided to hold back this information from parents until the last day of school.

“What's now required for those principals in those schools that have lost money, they now need to spend their holiday time rejigging staffing levels and rejigging timetables.”