But New Zealand were quick to overturn the early scare through tries from Sarah Hirini and Nathan-Wong to take a 12-5 advantage into the break. It was plain sailing from then on as the Black Ferns piled on four more on a tiring PNG side.
It was a similar story in their second game of the day as the Black Ferns took a 31-7 win over France, who they also beat 31-0 in Sunday's Fast Four final.
France were able to keep it close early after France's Chloe Pelle cancelled out Michaela Blyde's opening try.
But again, the Black Ferns pulled away in the latter periods as Stacey Waaka, Gayle Broughton and Tyla Nathan-Wong crossed over to put them well in front.
As the final hooter sounded, Blyde turned up again on the left wing, as she did all of last year, to score the final try of the game to put an exclamation mark on the win.
The Black Ferns ended a successful day by thrashing England 29-0 in their third game. They will play their quarter-final matchup on Saturday, with the semifinals and final concluding on Sunday.