New Zealand Prime Minister John Key has said he supports changing the national flag to remove Britain's Union flag, but would hold a referendum before any decision was made.

The current flag has attracted criticism because one corner features the symbol of its former colonial master, with the remainder consisting of four stars representing the Southern Cross constellation.

Mr Key said he believed the flag should display a silver fern, the national emblem already used by New Zealand sporting teams, such as rugby union's All Blacks.

"I'd like to see a change," he told reporters.

"But firstly it's not the single biggest issue that we as a country face. And secondly, even with those who want to change, there's not universal support for what we should change to.

"My personal view is, silver fern's the right way to go."

Mr Key said the New Zealand public was split 50-50 on adopting a new flag.

He said he planned to sound out senior ministers about the prospect of holding a referendum on the issue to coincide with a general election due later this year.

Mr Key said Canada, another former colony of Britain, had never regretted adopting its distinctive maple leaf flag in 1965.

"I know it was a ferocious debate in Canada, but in the end would any Canadians look back and say they got it wrong with the maple leaf?"