Key is a strong supporter of ditching the current flag, which features the Union Jack of former colonial power Britain in one corner, in favour of a more recognisably New Zealand banner.
"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," he said in a statement.
Key has previously said he would like to see a new flag featuring a silver fern, on a black background, similar to the banner already used by many New Zealand teams such as the All Blacks.
Key said Deputy Prime Minister Bill English would convene a cross-party committee covering the entire political spectrum to choose a number of potential designs for a new flag.
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The government will then hold a referendum in 2015, allowing the public to choose a preferred option from the range selected by the committee.
A second referendum is then planned for 2016, when voters will chose between the existing flag and the new design.
The existing flag was first used in 1869 and formally adopted in 1902. Its supporters say that New Zealanders have fought and died under it for generations and a change would dishonour their memory.
But critics argue it is too easily confused with those of other former British colonies such as Australia, which has an almost identical design.