"I think it's fair to say that this will be a wee one that a village will raise, but we couldn't be more excited," she also wrote.
Ardern said although there would be "lots of questions" she and her partner Clarke Gayford had a plan all ready to go.
And we thought 2017 was a big year! Clarke and I are really excited that in June our team will expand from two to three,...
Posted by Jacinda Ardern on Thursday, 18 January 2018
A baby due in June indicates the Prime Minister became pregnant in September – during the election campaign.
Ardern said in a press release yesterday that she learned she was pregnant on October 13 "but as many couples do in the early stages, we kept it to ourselves".
Winston Peters announced NZ First would form a coalition with Labour on October 19.
Ardern said she planned to take a six-week break from Parliament and Gayford would be the primary caregiver.
"Yesterday I met Deputy Prime Minister Winston Peters to share the news and to ask him to take on the role of Acting Prime Minister for six weeks after our baby is born," she said.
"As is the case when I am overseas, Mr Peters will act as Prime Minister, working with my office while staying in touch with me. I fully intend to be contactable and available throughout the six week period when needed."
Bay of Plenty MP Todd Muller said he was delighted for Ardern and Gayford.
"It will be a huge step in their lives together as it is for everyone who has children. I can still remember sitting with Michelle and our newborn and us looking at each other and saying 'now what' we felt seriously unprepared for parenthood, still do actually, but it's such an awesome journey – I wish them well."
Tauranga-based list MP Jan Tinetti said she was incredibly excited for the couple.
"What an amazing time for them. Babies are gifts and I can't wait to welcome this little one in to our Labour Caucus – he or she is going to have lots and lots of aunties and uncles."
I’m so excited for Jacinda and Clarke. Congratulations 😀
Posted by Jan Tinetti MP on Thursday, 18 January 2018
Fellow Tauranga-based list MP Angie Warren-Clark said the Labour Caucus was meeting on Sunday and Monday.
"So there's the opportunity to congratulate Jacinda in person and no doubt that will be the topic of the two-day meeting," she said.
"I'm absolutely delighted and congratulate them. This is a new phase of their lives together as a couple and they will be amazing parents and I am so very happy for them."
The Prime Minister's baby announcement made headlines around the world and her social media accounts were instantly flooded with messages of congratulations.
In response to the news, Tauranga MP Simon Bridges said children "are such a blessing".
"Natalie and I offer huge congratulations to Jacinda and Clarke on their very exciting news."
Tauranga-based list MP Clayton Mitchell also congratulated the pair.
"It certainly just goes to show what a vibrant and exciting new Government that we've got. I think it's a wonderful thing. Having three children myself I understand they'll be very excited."
The NZ First MP said Ardern would do a great job balancing a family and running the country.
"While she is away for the six weeks that she's announced, the country will be in great hands and the sun will still come up in the morning and with Winston Peters as the Acting Prime Minister I think it will be in good shape."
- Additional reporting NZ Herald
