PNG raced away to a 10-0 after only 15 minutes of play in the first half through a double from hooker Nathan Tente.
Behind on the scoreboard early in the game, NSW found their rhythm quickly and fought their way back.
The visitors found the try line 10 minutes out from half time when Jake Collins scored out on the wing.
A successful conversion further narrowed the score down to 10-6 going into the break.
Poor ball handling from PNG did no good for them as they gave away too much possession to NSW.
NSW capitalized on this and it wasn't long before this turned into points when Maleke Morris crashed over to score near the posts.
Jordan Allan made no mistake booting his side in front to lead 12-10 with 20 minutes remaining on the clock.
The game was destined to go right down to the wire and the score was tied at 12-12 after PNG's Amos John successfully kicked a penalty goal with 6 minutes remaining.
NSW had the last chance to win the match and did so with 20 seconds on the clock with a kick to the wing of Cody Ramsey who picked it up to score the winning try.
Meanwhile, the main game between the Australian PM's XIII and the NGCB PNG PM's XIII will kick off at 4:40pm.