WHS pulled clear again when they ran back a Cullinane clearance and Gabriel took the offload to dash away to the corner, and she soon had a hat trick after Diaz Gabriel ran from inside her 22m right to the tryline, with the ball eventually freed out to Hayley Gabriel to cross.
Under the pump at 20-7, Cullinane went to what they know, as powerhouse Waimarie Whanarere followed up a strong attacking sweep, which brought a tap penalty, to bash her way over.
Then Cullinane put passes through about eight sets of hands before it came back to Whanarere to smash under the posts for her double, with Hika's kick closing the gap to 20-19.
Yet WHS showed this wasn't the 2020 final, as a series of scrums and penalties got them back to the tryline, with Samara Pah-Long getting the recycled ball to score on halftime.
As the ball got more slippery, both teams struggled to keep possession, but WHS struck a crucial blow as Munro made a long run and Pah-Long had her own double off the ruck.
After Cullinane made the unfortunate error of an incorrect tap kick, WHS spread the ball from the scrum and Munro showed a clean pair of heels to outstrip the cover defence and then dash back towards the posts for an 80m stunner.
At 35-19, it appeared Cullinane were gone, but the defending champions dug in, finding gaps up the middle with their surges and offloads, as Whanarere finished off a movement she started to score her hat trick.
The home side's speedsters got free down the left hand touch to make it back to the tryline, with Raycena Baron diving full stretch from the ruck to make it 35-29 as the clock ticked away.
But WHS didn't panic as they kicked for territory and forced Cullinane to try to work out from their 22m, and despite a strong surge back up-field, eventually the ball was turned over.
The visitors had the final attack and Pah-Long produced a great offload for Diaz Gabriel to dive under the posts on fulltime.
Whanganui High School 40 (H Gabriel 3, P Munro 2, S Pah-Long 2, D Gabriel tries) bt Cullinane College 29 (W Whanarere 3, T McKenzie-Mason, R Baron tries; H Hika 2 con). HT: 25-19.