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."