Jacinda Ardern is only the second elected leader to give birth while in office.

New Zealand Prime Minister Jacinda Ardern said on Monday she was surprised when her long-time partner proposed to her over the Easter holidays and has no idea when the wedding will be.

PM Ardern was addressing her first weekly media briefing after news of her engagement with fishing-show host Clarke Gayford became public last week.

"I was surprised by the question," PM Ardern said about the proposal, adding the couple haven't made formal wedding plans.

PM Ardern said the proposal was made when the couple, plus her diplomatic protection security, some locals, and a dog who stole her chocolates, were at a "beautiful little lookout" on Mokotahi hill on the Mahia peninsula in Hawke's Bay, New Zealand.

"So, it was very romantic...," she joked.

The forthcoming nuptials are a rarity for world leaders in office and follow Ardern's pregnancy last year which was seen around the globe as a symbol of progress for female leaders.

Ardern is only the second elected leader to give birth while in office, after Pakistan's Benazir Bhutto in 1990 and, if she marries while in office, will be the first major leader to do so in recent years.

Her fiance; is a 41-year-old host of a television fishing show who takes care of their 10-month-old daughter Neve Te Aroha, while PM Ardern, 38, runs the country.

News of their engagement broke after journalists noticed PM Ardern wearing a ring on her middle finger at a public event on Friday. PM Ardern said it was Gayford's grandmother's ring.

"I have not being trying to hide our news from anyone, but it simply does not fit on the right finger so that's why its sitting in the middle," she added.