Finance Minister Bill English says the growth of Government assets, especially the ACC investment fund and the so-called Cullen super fund, could have an unpredictable effect on the forecast surplus.
The Government's half-yearly accounts, to be unveiled on December 17, would still forecast a small surplus and the Government was still committed to achieving it in the 2014-15 financial year, he told reporters at Parliament yesterday.
But the fact that the funds were so big meant that small changes, for example to interest rates, could have a big impact.
Over the past five years the ACC investment fund had increased from $9.6 billion to $24.6 billion and the New Zealand super fund had increased from $14 billion to $23 billion (even with Government contributions suspended).
"The ownership of this wide range now of financial instruments through the investment funds and the big liabilities associated with the earthquake just make it a bit trickier."
He said Government revenue and spending was "in pretty good shape".
"But the accounting impact of this now large growth of financial assets is a bit unpredictable.
"When you are aiming for a very small difference between large numbers - the surplus [forecast] in the Budget was around $70 million on a spending base of $70 billion - you are trying to land on a penny in the middle of a storm.
"So the accounting effect on all of those assets can create uncertainty."
The funds themselves are not calculated in the surplus - because New Zealand's surplus, or deficit, is measured by the operating balance before gains and losses.
"But the way they change in value and makeup can have an impact and because they are now so big, much bigger than the value of SOEs, for instance, they can kick your surplus around."
Just because there was confidence about growth in the economy, people should not be expecting some large unexpected surplus windfall.
Changes in the value of the funds could impact on the tax they paid.