Speaking to media this afternoon, Ardern said no matter someone's history, if they possess images which portray children being abused that "undoubtedly completely rewrites your history. Undoubtedly."
Brierley wrote to the Clerk of the Executive Council to tender his resignation as a Knight Bachelor.
Queen advised of forfeiture
"The Queen has been informed," Ardern said.
This means he's no longer allowed to use the title of "Sir" and he has been asked to return his insignia.
Officials wrote to Brierley on April 6, telling him he had 30 days to provide any information that he considered relevant before the Prime Minister made her decision.
Ardern said after he was notified the process was under way, Brierley resigned the title.
"But I'm very clear - had he [not] done so it would have been removed."
Last month, in an Australian court, Brierley pleaded guilty to three charges of possessing child sexual abuse material.
His barrister admitted he possessed "some images" but the exact figure is "in dispute".
According to the Guardian, he faces a maximum of 10 years in prison when he is sentenced.
He is 84 years old.
"I think it's a sad day for the children of New Zealand and, indeed, the world when someone is found guilty of possessing such horrendous images," Ardern said.
She added that it was only right that there are "significant consequences" for that.
As a country, she said, New Zealand has to reject anyone who thinks this is okay.
Wellington College, where Brierley attended high school, severed its ties with the disgraced businessman after his guilty plea.
Although the process of stripping a knighthood, or damehood, is not unprecedented, it does not happen often.
Albert Henry, the first Premier of the Cook Islands, lost his knighthood after he was forced to resign in a 1978 voting scandal for which he was later convicted of fraud.
Hugh Hamilton and Morgan Fahey also had honours removed - in each case having their appointments as Member of the New Zealand Order of Merit revoked.
