The government's budget surplus for the five months ended November was running slightly ahead of forecast due to it collecting more tax and incurring fewer expenses than forecast.

The surplus of $261 million was in contrast to Treasury's expectations of an $88m deficit for the five months.

Tax revenue in the five months was $141m higher than expected at $32.9 billion while core Crown other revenue was $61m more than expected at $35.9b. Core Crown expenses were $409m less at $35.52b.

Treasury says the higher tax revenue was due to higher than expected deductions at source and GST, offset by a lower corporate tax take.

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The lower-than-expected expenses mostly reflect underspending on the government's social housing and KiwiBuild policies, which Treasury says are likely to be timing differences.

Gross government debt of $88.15b at November 30 is $504m less than expected and 30.3 per cent of GDP, with net debt of $61.98b being 21.3 per cent of GDP.

Treasury is forecasting a $1.72b surplus for the full year ending June.