Prime Minister Jacinda Ardern has been given a big welcome at a very family-friendly Labour Party caucus retreat in the Wairarapa.

Many MPs have brought their spouses and children to the first caucus of the year, near Martinborough.

The Prime Minister's partner Clarke Gayford is one of those in attendance.

However, Ms Ardern said inviting family to this event is common for Labour - and not connected to her news that she'll be having her first baby in June.

"In fact, we have for a long time invited family members to the beginning of our political year," the PM said.

"Acknowledgement that actually, it's our family members who often don't get to interact with everyone else and so it's a really nice opportunity to bring everyone together - so, this isn't a first."

Ms Ardern told reporters that because there is no way to adjust the pay of an MP away, she will be donating part of her salary to Plunket for the time she takes off following her pregnancy.

Updating the list of well-wishers on her baby news, Ms Ardern said British Prime Minister Theresa May has sent a text, as has Australian Labor leader Bill Shorten.

Canadian Prime Minister Justin Trudeau earlier tweeted his congratulations.

The Labour caucus is not expected to make any decisions at the retreat, but it will be briefed on the remaining parts of the 100-day plan, which is up on February 3rd.