They and their colleagues were in Napier for what should have been a routine annual caucus retreat prior to the House resuming, but instead they are now preparing for a leadership vote.
Dunedin MP Dr David Clark once shared a bench in the House with Ms Ardern and also once had an adjoining office.
Having also endured his own turbulent moments in the public spotlight, he was understanding of her surprise resignation.
"It is very difficult for politicians, the toll the job can take on our families and the sacrifices that they make, more than the politicians themselves ... I completely understand her decision to focus elsewhere because of the demands of her role.
"History will reflect that she has taken on some huge challenges and has responded with empathy and compassion and an instinct that I think reflects the best of who we consider ourselves to be as New Zealanders."
Having served Ms Ardern as a Cabinet minister, Dr Clark said that she was an exceptional chairwoman and had effectively communicated Labour’s policies on every issue.
"She brings that trademark compassion to the decision making process and I haven’t encountered anyone else who brings all those skills together."
Dunedin list MP Rachel Brooking said she had been totally surprised by Ms Ardern’s announcement, but took immense pride in the Prime Minister’s achievements, especially in the area of climate change.
"She has worked so hard and she is only one person and, as she said, she is only human ...
"If she felt she did not have enough in the tank to do another three years, I totally respect her decision."
Taieri MP Ingrid Leary said it had been an incredible privilege to have worked with Ms Ardern.
"It [her resignation] was a personal decision based on what she could give to New Zealand and once again she has done it with absolute integrity and I have nothing but admiration and respect for her."
Ms Ardern had told the caucus they could win the election and she and her colleagues were very confident they could win, Ms Leary said.
"I don’t think that she would have done this if she didn’t think that we could win, but her decision has been about what is best for New Zealand and the best for herself."
Invercargill list MP Dr Liz Craig said Ms Ardern had been an amazing leader and inspirational person to work with.
"The last five years have been incredible and she has had to be decisive, but her empathy has been amazing and she has made a huge contribution."
Looking ahead, there was considerable talent in the Labour caucus, but for the moment she only wanted to reflect on Ms Ardern’s leadership, Dr Craig said.
Labour MPs will vote on Sunday for its next leader and the country’s new prime minister.
