Reasons for Actual Budget Outcomes
In most cases, actual budget outcomes are not the same as budget estimates. This can be contributed to a range of factors such as:
Budget measures do not pass through both houses of government or are modified - for example, the 2016/17 budget included estimated revenues from a 32.5% backpacker tax which was compromised to only 15% from political pressure
New policies are introduced and passed in both houses in government to which the budget has not been considered.
Economic estimates were wrong - budget estimates are based off predictions in future economic indicators. Actual economic indicators are often different from budget estimates, leading to changes in revenue and expenditure.
Unforeseen event - a natural disaster may require the government to contribute government spending to assist in affect areas. This expenditure is unplanned and not included in government budget estimates.
Changes in the exchange rate
Changes in terms of trade