New Zealanders will vote twice over the next two years on whether the country should change its flag, the government has said.

Next year, voters will choose their favourite alternative flag from among several options, and in 2016 they will vote on whether that alternative should replace the current flag, the government said.

Some in New Zealand consider the current flag to be an unwanted relic from a colonial past and too similar to Australia's flag. Others, including many combat veterans, feel a deep attachment to it.

The current flag features the Southern Cross star constellation with Britain's Union Flag in the canton, or top left corner.

Prime minister John Key favours a change and promised to hold a vote if he was re-elected for a third term in September. He won a decisive victory in the election.

"Our flag is the most important symbol of our national identity and I believe that this is the right time for New Zealanders to consider changing the design to one that better reflects our status as a modern, independent nation," Mr Key said in a statement.

When Mr Key first raised the idea of a flag vote in March, he said he favoured changing the design to a silver fern set against a black background, an image that is popular among New Zealand's sports teams, including the national rugby squad, known as the All Blacks.

The government said several alternatives will be chosen for next year's vote by a panel of "respected New Zealanders" who will be nominated by a group of lawmakers from all political parties.

Mr Key estimated the two votes would cost 26 million New Zealand dollars.

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