The international rating agency announced its decision by saying the nation enjoys monetary and fiscal flexibility and a resilient economy that offset the risks of high external debt.
‘‘The short-term rating was affirmed at A-1+. The stable outlook reflects S&P's expectation that New Zealand's fiscal performance will improve over the medium term, offsetting vulnerabilities associated with its high external debt''.
‘‘We believe that the checks and balances in government are effective, and New Zealand ranks near the top of many surveys on governance, government efficiency, and business promotion,'' S&P said.
‘‘The Reserve Bank has strong credibility regarding its inflation mandate and its supervisory role.''
The nation benefited from a high-income and resilient economy, which S&P believed drew from decades of structural reforms and wage restraint, it said.
Mr English said the Government's responsible and economic fiscal settings, economic resilience and Reserve Bank credibility were all highlighted by S&P.
‘‘That shows why we need to continue with consistent and responsible economic policy.
"Despite ongoing volatility internationally, S&P expects the New Zealand economy to grow by an average of 2.7% over the next four years.''
One risk highlighted was New Zealand's level of foreign debt, he said.
Net external debt had fallen to 55% of GDP, down from 83% when National took office. It was at its lowest level since 2003.
The best thing the Government could do to reduce national debt was to get its own finances in order. That was why the Government had been so focused on turning the books around from an $18.4 billion deficit in 2011 to a small surplus, Mr English said.
The Treasury was forecasting New Zealand's core Crown net debt to be 27% of GDP in 2015-16 before falling to about 20% of GDP in 2020. That compared to current net debt levels of about 18% in Australia, 38% in Canada and 80% in the United Kingdom and the United States.
New Zealand also had a top Aaa credit rating from Moody's and was rated AA by Fitch.
