The forecast Budget surplus for the year to June 30 2015 will be small and exposed to the influence of even small changes in the value of some government liabilities, particularly government bonds, says English.
A bigger corporate tax take and less spending on the Canterbury earthquake than expected.
On the nation's current track, revenue will fail to meet expenses by the mid-2020s, Treasury says.
Investors offered to buy more than six times the amount of bonds on offer in the NZ Debt Management Office's first sale of the securities this year.
The Reserve Bank says the government's efforts to build its revenue base will slice four percentage points from nominal GDP over the next four fiscal years.
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