An independent watchdog says there are "significant risks" to the federal budget from spending rises and revenue falls.
The government's projections show the budget moving from a deficit of 3.1 per cent of gross domestic product to a surplus of 1.4 per cent of GDP in 2024/25.
However, Parliamentary Budget Officer Phil Bowen said there was a "reasonably uncertain international outlook" that would affect revenues.
"That is not helped by international conflicts and of course concerns about the Ebola epidemic," Mr Bowen told a Senate estimates hearing in Canberra on Monday.
The International Monetary Fund and World Bank had downgraded their forecasts of world economic growth including Australia's key trading partner, China.
Mr Bowen said the budget papers had allowed for a further decline in the terms of trade.
"But there's always the risk that commodity prices could fall faster and this would have a negative impact in the short term on the budget."
There were also revenue measures not yet passed by the parliament, the most important of which was the reintroduction of regular fuel excise rises.
On the spending side, Mr Bowen said there was a community expectation the government would increase levels of services over time.
"At the end of the period there is a surplus projected but it is based purely on the current policy settings without net additional spending," he said.
At least $18 billion in budget savings measures have yet to be legislated.
Mr Bowen said the surplus would inevitable shrink if the measures were not passed, by as much as one per cent of GDP.
"We are not saying the position is unsustainable, but we are raising the prospects of risks to sustainability," he said.
"I'm not crying wolf or trying to inject a flavour of panic - I am simply talking about the need for sensible budget management which does include building a sensible buffer against such shocks."
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