The Prime Minister has been accused of handing out threats and using scare tactics to try to frighten people into choosing the alternative flag.
John Key warned if people voted against changing the flag they would not get another chance until New Zealand became a republic - which he said would not happen in his lifetime.
On Thursday, the voting packs go out, which gives people the option to choose between the current New Zealand flag or the new black and blue Kyle Lockwood design.
Mr Key said if the alternative flag was rejected, a few months from now people would regret it.
"If they don't vote for change now, they'll never get another chance until we become a republic.
"Personally I don't think that's going to happen within my lifetime, because I think the young royals who are coming through are much-loved - and, actually, the royal family has done a very good job of cementing themselves as our constitutional head of state."
But New Zealand First leader Winston Peters said that sounded like a threat.
"It sounds like he's panicking. He's actually using sort of fear tactics, saying 'this is your last chance if you don't change the flag'," he said.
"But you know, when you're running with a fetish of a change and the people don't agree with you, including half the people in your political party, then you're on a hiding to nothing frankly."
He said the royal family had nothing to do with changing the flag.
Labour leader Andrew Little said it would not be such a bad thing if New Zealand did end up waiting to bring in a new flag.
It was better than choosing a flag no-one likes, he said.
"The problem is that if New Zealand adopts a flag that some people compare to a tea-towel or a sports strip, we're going to be stuck with that for 100 years," Mr Little said.
"By not accepting the new design flag, we leave the way open, for a reasonable period of time - 10 years or 20 years - to adopt a new flag that actually most people can get behind and support."
A leading pro-republic campaigner, and the man behind the Change the NZ Flag campaign, Lewis Holden, agreed with Mr Key that it was the last chance to change the flag unless New Zealand became a republic.
But the former National Party candidate said the topics should be kept separate.
"The two issues are linked in the sense that they're about national identity but I think with this flag referendum as well, we have to consider that it's really better that these two issues are kept separate rather than trying to deal with both of them at once."
Mr Holden said the real problem was the flag debate had become too political.
People have until Wednesday to enrol and the official voting period starts on Thursday and ends on 24 March.