She told reporters that because there is no way to adjust the pay of an MP, she will be donating part of her salary during that leave period to Plunket.
"There is no mechanism for MPs to have their salary change in any way [but] I'd like to give a donation to Plunket because of the fact that whilst I'll still be present, my job will be downscaled for the six weeks that I'm away."
Updating the list of well-wishers on her baby news, she said British Prime Minister Theresa May sent a text, as had Australian Labor leader Bill Shorten.
Canadian Prime Minister Justin Trudeau tweeted his congratulations.
The caucus is not expected to make any decisions but be briefed on the remaining parts of the 100 day plan, which is up on February 3, which she said was on track.
There are four outstanding pledges - to begin inquiries into abuse of children in state care and into mental health, to introduce legislation setting up a framework for child poverty reduction targets and monitoring and to introduce legislation for workplace reforms.
Asked about concerns by employers around the workplace legislation, she said: "We always keep our dialogue with business open, transparent and engaged.
"What I hope will happen is that after they see a bit of confidence amongst their consumer base, that we might see that change over time."
The Government recognised that when the economy was strong and doing well, everyone benefited.
"So it is just about proving that. I accept that is going to take a little bit of time."
Ardern will be handing the reigns of Prime Minister to Deputy Prime Minister Winston Peters when she takes leave, but she said there would be no need to make Kelvin Davis acting Labour leader under Labour's constitution because she will still be in the country.
Asked about the view of some people that she couldn't be both a mother and Prime Minister, she said she knew and appreciated that view.
"All I can do is give the assurance that I take my role as Prime Minister incredibly seriously and that I will be fulfilling the mandate that I have.
"I expect that people in some cases will simply wait to see if I perform up to their expectations.
"And you know, if my grandmother was still here, she might be one of the people expressing that view."
She accepted that there was a whole range of takes from individuals.
"I don't take that personally and nor do I think I should. I've just got to get on with it prove to people that I can do the job."
Ardern's grandmother died during the election campaign in September.
