Former Prime Minister Sir John Key revealed his biggest regret was not being able to change the New Zealand flag.

In an interview with the Australian Financial Review, Key told Patrick Durkin it was something he had often thought about.

"The reason I wanted to change the flag was to have something that was uniquely New Zealand, so that we could build a more overt sense of national pride," Key said.

"This is not the most important issue by a long way for the country, but ultimately I was unsuccessful in changing the New Zealand flag and I have always thought back on that."

Key said it didn't pass because Labour and the Greens decided to turn it into a political issue and made it a referendum on him.

"It is not going to change the world and there are many things we did, from the Christchurch earthquakes to economic issues that we dealt with, but that is the one I feel I definitely failed on and left on the table," he said.

"I suppose I also have to take responsibility and say, well look, maybe there were some other things I could have done. To be frank, I think I should have just pushed it even harder."

Key said Kiwis were typically more reserved than in places like the United States.

"Americans use the flag to symbolise what they believe, which is that America is the greatest country in the world," he said.

Key spoke about how his father died when he was six, and how his mother raised him in very poor circumstances.

"She was an Austrian-Jewish refugee who got out just before the Nazis invaded in 1938," he said.

"My mother was a very determined Jewish matriarchal woman. She always used to say, 'You get out of life what you put into it'."

His advice to people, especially the younger ones, was that if they wanted to do something then they themselves had to make it happen.

"I wanted to be prime minister, I wanted to be financially successful, I wanted a successful marriage," Key said.

"A lot of times the reason why people don't act is the fear of failure; they are worried about what other people will think if they don't quite make it. I say to people, 'Just back yourself'.

"In the end, I would rather give it a go and fail then sit back and be a Monday morning quarterback critiquing everyone else."