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.