The surging New Zealand dollar has helped to push the Government's books deeper into the red.
Gross debt at the end of April was $2.9 billion, or 4%, higher than forecast at the time of the Budget in May.
The Treasury says the surging kiwi has increased the value of the Government's investments.
But that has required the Treasury to request more money from borrowers as collateral, which is recognised as a liability on the Government's books.
That boosted the Crown's gross debt, although net debt was unchanged from that forecast in the Budget.
The Treasury also says revenue in the 10 months to the end of April was $80 million lower than Budget forecasts.
However, that was offset by spending being $500 million lower, resulting in the operating deficit being $400 million smaller than forecast.