Peter Dunne wants to have a referendum in 2020 on New Zealand becoming a republic, to be held before the term of incoming Governor-General Dame Patsy Reddy expires in 2021.
The United Future leader said that a five-year time frame would give it more time than the flag referendum, which had squeezed in two votes in 18 months.
It would a clean vote on a simple question.
The question would be something along the lines of "should New Zealand become a Republic within the Commonwealth with an elected President as head of state?"
"You'd have a five-year term, just as the Governor-General does now."
"It's a clean swap, if you like. It's not difficult," he told the Herald.
Legislation would be passed in advance to allow the election of a president in 2021 after Dame Patsy's term if the answer to the referendum in 2020 was yes.
"If the answer is No, nothing changes."
He said Ireland provided a good model, with a parliamentary system and an elected president as head of state.
"The elected president does not exercise any executive functions and is obliged to act on the advice of his or her ministers, in pretty much the same way as our Governor-General does now.
The difference as that the president in Ireland was directly elected by the people, and was not the representative of a foreign hereditary monarch at the other end of the world, as New Zealand Governor-General was.
Mr Dunne proposed the referendum in his weekly bulletin, "Dunne Speaks" and said the most extraordinary feedback he received was that such a referendum should wait until after the current head of state, the Queen, dies.
"I find that a little bizarre. If the best reason for doing it is that the Queen has died, you might as well forget it now. It has got to much more positive than that."
Mr Key this week confirmed that Dame Patsy Reddy would be the next Governor-General and her term begins on September 14.
Mr Dunne said he was in favour of the two-stage referendum for the flag process and he said the process had gone a lot better than many acknowledged.
"But I think it has probably been a little rushed. The arrival of a new Governor-General gives us a clear five years."
He said his comments were made with no disrespect to Dame Patsy.
"She is another outstanding selection in that now long line of impressive New Zealanders to hold the office, and I have no doubt she will do a superb job and quickly earn the respect of New Zealanders.
But the appointment provided a chance to think afresh about New Zealand future constitutional structure.
"I have long believed New Zealanders are ready for that discussion."