Prime Minister John Key isn't keen on a black background for New Zealand's next flag, but likes the idea of a silver fern.

Prime Minister John Key has backed off black for New Zealand's new flag but remains keen on the silver fern for the ensign design.

He says losing the black is more a fashion choice than a reaction to the Islamic State (IS, or ISIS) black-backed banner.

Key raised the flag issue at a breakfast at Waitangi following today's dawn service at Te Whare Runanga, marking the 175th anniversary of the signing of the Treaty in 1840.

He told leaders he wanted to see a new flag by 2040, the 200th anniversary of the Treaty, in marked contrast to Fiji Prime Minister Voreqe Bainimarama's decision to scrap the Union Jack on his flag by the end of this year.

"He has got an instant process, no referendum for him, he is just deciding to do it and he's changing the flag," Key said.

"We are going through a much more considered process."

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Key said the public would decide on what the new New Zealand flag would be and that could include having a black flag if it was voted for.

"I think there will be some people who say black is not the most colourful for a flag. Some will say it is too close to ISIS."

Key said he would hate to think Islamic State drove anything in New Zealand "other than a galvanising view that we need to stand up against brutalising people."

To him the important consideration was the silver fern on the flag.

"It is the symbol of New Zealand, it is internationally recognisable.

"If you go to the gravestones on the Western Front, for instance, those brave New Zealand men buried there have the silver fern on their tombstone."

Key said people wanting to remain with the current flag often had that view because they did not know any alternatives. They would be given some and would probably like the change.

He compared it to deciding on whether to keep the old car or get a new one.

Key said he received hundreds of submissions on a new flag and "there are some damn good ones".

Claiming he had no final preference he added "I am going to wait and see what comes up."

Key said other than changing the flag, he remained a constitutional monarchist and did not want to see New Zealand becoming a republic.

In his speech Key said the current flag represented the thinking of a young country moving from the 1800s to the 1900s.

"Our role in the world was very different then. Our relationship to the rest of the world has changed over time."

The flag captured a colonial and post-colonial era whose time has passed.

Key noted that Maori chose New Zealand's first flag, the United Tribes ensign in 1834.

"Three flags were displayed on short poles at Waitangi, voted on, and the winning one hoisted," Key said.

"I imagine it was all over in a matter of minutes. The process this time around will be much more considered, but I have every expectation Maori will be closely involved, just as they were in 1834."

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