Government revenue in the six months to December 31 was $743 million lower than forecast, Treasury accounts released on Monday show.
Tax revenue was $400m less than anticipated, with GST $234m below forecast.
The deficit stood at $9.5 billion, which was $2.6bn higher than forecast.
Treasury says this was mainly due to actuarial losses on the Government Superannuation Fund, outstanding ACC claims and higher than forecast investment losses.
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