Prince Charles is strange and his father so insensitive and prejudiced that he could be a breakfast TV host, says New Zealand's former deputy prime minister.

Michael Cullen's comments, contained in notes for a speech he will make in Wellington this week, are bound to outrage supporters of the monarchy.

As a senior Cabinet minister, Cullen described himself as the Labour Government's "token monarchist" and fought against any move for New Zealand to become a republic.

But, in a major about-turn at a constitution conference on Friday, he will publicly lay out a road map to becoming a republic when the Queen dies.

He will also propose that New Zealand adopt a tino rangatiratanga flag as its own, albeit in red, white and blue to recall the country's British heritage.

In his speech notes, obtained by the Herald on Sunday, Cullen says Prince Charles (next in line to the throne) and his father, the Duke of Edinburgh, "evince some strange characteristics".

"But talking to plants and extremely conservative views on architecture are scarcely confined to Prince Charles.

"And as for the old Duke, his insensitivity and prejudices would in our nation make him highly suitable material for a breakfast television or talkback radio host."

He says far better reasons for appointing our own head of state are that the monarchy in Britain seems less relevant to young Kiwis and that the preferment for male heirs and Anglicans is inconsistent with modern New Zealand values.

Echoing the views of Australian prime minister Julia Gillard, he says Queen Elizabeth II should be allowed to live out her time as Queen of New Zealand because of her "dogged" and "old-fashioned" sense of duty and determination to do what is right.

But New Zealand needs to make a start on law changes so that we are ready and able elect our own head of state, "on the death or incapacity of the Queen".