News of the couple’s engagement emerged after Ms Ardern was seen at a ceremony on Friday wearing a diamond ring on the middle finger of her left hand.
A hawk-eyed journalism intern spied the new addition and asked the prime minister’s office about it.
Her spokesman then confirmed that the pair got engaged over Easter.
Last year, Ms Ardern gave birth to the couple’s first child, a daughter named Neve Te Aroha.
Earlier this January, Ms Ardern was asked by the BBC’s Victoria Derbyshire if she would ever propose to Mr Gayford.
“No I would not ask, no. I want to put him through the pain and torture of having to agonize about that question himself,” she said.
Ms Ardern was the second world leader to give birth while in office. The first was the late Benazir Bhutto, Pakistan’s two-time prime minister.