"This is wonderful news. On behalf of the Green Party, I'd like to congratulate Jacinda and Clarke, and wish them the very best for parenthood."
National Party Leader Bill English also extended his congratulations.
"Mary and I would like to extend our sincere congratulations to Prime Minister Jacinda Ardern and her partner Clarke Gayford on the news of the Prime Minister's pregnancy.
"It is an incredible privilege to be a parent. A new child will bring real joy to their lives. We wish them all the best."
English himself spent time as a house husband while his wife Mary worked.
"I did it in the late 80s, out of the suburbs, actually, where a male out in the suburbs during the day was regarded as unemployed or had some kind of disability on a benefit," he has said of his time as a house husband.
New Zealand First leader Winston Peters will be Acting Prime Minister for six weeks while Ardern takes six weeks with her new baby in June and July.
"On behalf of my New Zealand First colleagues we wish the Prime Minister and her partner every joy during the next several months as they adapt to a new challenge," Peters said.
Ardern said that Peters has said he will prioritise his duties as foreign minister to make sure he is available over the period he will be Acting Prime Minister.
Peters said New Zealand First would continue to contribute to a stable Government in the coming months.
"Ours is a novel coalition in many respects and the Prime Minister's announcement only reinforces its unique qualities.
"We are committed to supporting the Prime Minister during the months ahead to continue to provide a good and stable government. We understand that no matter what level a person operates on family life is important."
Former Labour leader Andrew Little said: "Fantastic news ... Congratulations boss!"
Act leader David Seymour said: "Of course we are political opponents, but congratulations to [Jacinda Ardern] on her wonderful news."
Even former prime minister Helen Clark took time to celebrate the news saying, "She's a great role-model nationally and internationally ... I doubt there is a female leader of any country that has had a baby while in any office."
- Additional reporting NZN