Fitch Ratings said on Thursday that New Zealand's budget surplus "underscores the positive outlook on the sovereign's 'AA' rating".

The New Zealand government reached its budget surplus in the fiscal year ending June 30. It was the first budget surplus since 2008. The budget surplus was driven by stronger-than-expected tax revenue.

"Fitch believes that some of the improvement in revenue may be sustained into FY16. But the outlook for the dairy sector, and the implications for the wider economy, will be an important determinant of revenue growth," the agency said in its statement.