The government recorded a $2.3 billion surplus in the first 11 months of the financial year.
But the Treasury warns that when the final numbers come through for the year to the end of June the surplus, excluding investment gains and losses, will not be as strong.
It says a surge in spending in the last month of the financial year is expected to exceed the government's income.
That should mean the final surplus for the year to the end of June, once investment gains and losses are removed, is closer to the December forecast of $668m.
In the 11 months to the end of May, not only was the surplus better than expected, but the government's net debt was also $469 million lower than forecast.