Five months into the Government's financial year the operating deficit was $4.48 billion - or $250 million more than forecast in the pre-election economic and fiscal update.
It was mainly due to tax revenue falling $500 million or 2.3 per cent short of forecast, the Treasury said.
Source deductions (mainly PAYE) were nearly $400 million less than forecast and GST just over $300 million less, but the corporate tax take was a positive surprise at just over $200 million above forecast.
"The source deductions and GST revenue variances were mainly timing-related and are expected to reverse," said the Treasury's chief financial officer, Fergus Welsh.
"However, there is a risk that some of the GST variance may not reverse by year end."
The corporate tax take appeared to be due to higher-than-expected corporate profitability, but gross domestic product in the third quarter was lower than the Prefu forecast, suggesting profitability might be lower than forecast by year's end.
"So overall, there is a downside risk to tax revenue for the current year," Welsh said.
The cash deficit for the five months to November was $7.5 billion, $280 million bigger than forecast.
Finance Minister Bill English said the revenue shortfall reinforced the need for ongoing spending restraint and responsible fiscal management.
"The Government is committed to ... returning to surplus in 2014/15," he said.
Returning to surplus and repaying debt were among the most important things the Government could do to ensure New Zealand could withstand future shocks and build a more competitive economy.
English said the Budget Policy Statement on February 16 would confirm the Government remained on track to post a surplus in 2014/15.