The Kiwi Ferns were error-ridden in slippery conditions at CBUS Super Stadium to only lead by two at half time thanks to a Kaiyah Atai try right before the break.
But New Zealand flexed their muscles in a dominant second-half, running in five tries in the second 40 to secure a comfortable win heading into next weekend's semi-finals.
Mackenzie Wiki and Abigail Roache scored doubles for the Kiwi Ferns while fullback Apii Nicholls recorded 251 running metres in a player of the match performance.
The Orchids were arguably the better team in the opening half, holding a 12-8 lead thanks to tries from Tia Molo and Elsie Albert.
But PNG couldn't match that intensity in the second half as the more-fancied Kiwi Ferns surged away to an ultimately comfortable win.
Kiwi Ferns captain Raecene McGregor said her team will need to lift their performance going into their semi-final against Samoa.
"We need to be better, especially coming into the semi-final next week," McGregor told the Seven Network.
"Once we clean up a few of those errors and stick to the gameplan, probably execute a bit better, we'll come away with the result."
Earlier on Saturday, Fiji made history by thumping France 54-6 to set up a semi-final against Australia - reaching the last four in their first Women's Rugby League World Cup appearance.
The Buli Kula ran in 10 tries to one to record their second victory of the tournament and ensure they would finish in second spot in Pool B behind New Zealand.
Winger Tahlia Marshall scored a double for Fiji while Latisha Smythe was named player of the match following a dominant forward display.
"It's going to be a really hard game but for us the focus is just on us for the week ahead. We're not really thinking about the Jillaroos too much," Fiji's Cassie Staples said.
"We need to make sure we minimise those errors and take advantage of the opportunities we get out on the field."
