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Becoming a republic would be the "making of New Zealand as a country" and we need to start talking about it now, according to Labour leader Phil Goff.

In the most direct call for change from any senior politician yet, Goff said the end of Queen Elizabeth's reign was the logical transition point.

"Succession of the monarchy is the time to have a head of state who is a New Zealander," said Goff. "We need to start the conversation now. Don't rush it. Fully consult the people of New Zealand. It's a major change and needs a reasonable consensus."

Prime Minister John Key has previously said it was "inevitable" New Zealand would become a republic but has ruled out the move coming during his time as prime minister.

The Herald on Sunday revealed last week that former deputy prime minister Michael Cullen - who once described himself as a monarchist - had switched his support in behind appointing a New Zealander as head of state.

Cullen indicated that the monarchy seemed increasingly irrelevant to young New Zealanders.

One of Britain's biggest newspapers, The Daily Mail, reported Cullen's scathing criticism of the Duke of Edinburgh and Prince Charles, who is next in line to the throne.

It described his speech "the most personal attack yet by a prominent New Zealand figure on the British Royal Family".

Now, Goff has told the Herald on Sunday that when he was foreign minister and trade minister, people he spoke to, particularly in Asian countries, were often confused that New Zealand's head of state was from another country.

He said discussion about becoming a republic would be a lengthy one that dealt with issues around the Treaty of Waitangi and the flag.

"Let's begin a formal process now," said Goff. "Where do we want to be in 20 years?"