Rodney Hide makes an excellent point on his blog. The official surplus projection for 2008/09 is $5.3b. But this includes an allowance for extra spending of $1.9b per year cumulative. This means on current expenditure one in fact is looking to have an $11b+ surplus in 2008/09.
Now it is sensible to have some allowance for additional spending initiatives but there is nothing magical about $1.9b/year cumulative. It used to be $600m a year.