Played in extremely hot conditions, the big Warriors side were expected to wilt in the Sydney sun as the game wore on but a powerful surge to start the second half gave them the buffer they would eventually need to win a third under-20s title in the space of five years.
The Warriors only scraped into the finals at all courtesy of a Mason Lino sideline conversion that earned his side a draw in Round 26 against the Panthers and have been on a giant-killing run to claim the title.
"They're an outstanding bunch of boys, we got momentum and we knew that if we got to the eight we'd give it a good crack and we did the job," said Warriors coach Stacey Jones.
"Obviously the Broncos, when they had the ball they got to the end of their sets and put us under pressure with their kicking game and we just couldn't get ourselves out of it. We just needed to get our hands on the ball and to our boys' credit they defended real well at the end."
A try three minutes before half-time enabled the Warriors to take a 22-6 lead into the break and when Ken Maumalo and Sam Lisone scored inside the first 12 minutes of the second half a 34-6 lead seemed insurmountable.
But, as can often be the case in under-20s, the Broncos didn't see it as a mountain too high and launched an astonishing comeback that only ended when a Jayden Nikorima conversion of his own try from wide out in the final minute was waved away and the Warriors held on to claim victory.
Despite the crushing disappointment of his missed shot at goal, Kodi Nikorima said he was proud of his little brother for the way he dragged his team back into the contest on his 18th birthday.
"Proud of him, without him we probably wouldn't have been in that situation," Nikorima said. "I just told him to keep his head up and how proud I was of him. To score that try was hard enough as it is and it was always going to be a pressure kick.
"He loves those moments. Any moment like that he'll step up and take it, that's just the type of person he is.
"When we were 10 points down and there were 13 minutes to go I knew we were a red-hot chance. We're a second-half side so we knew eventually we'd come over the top, I guess we just let them get away too much."
Brisbane winger Paul Byrnes launched the fightback with a storming effort coming back on the angle from his left wing, powering through a handful of Warriors defenders but that paled in comparison to his second two minutes later that came courtesy of an Ashley Taylor bat-back of a kick that was well beyond the dead-ball line by the time he got his right hand to the ball.
Warriors second-rower Michael Ki was denied a try for the second time in the game by the referee – must to the surprise of coach Stacey Jones – when he was ruled to have knocked on close to the line despite the suggestion of a Broncos player knocking the ball free from his grasp.
The 16-point deficit was reduced to just 10 with 14 minutes to play when Kodi Nikorima sliced through to score and younger brother Jayden converted as the Broncos continued to charge home.
From the next set the Broncos again powered down-field and some spectacular play on the right culminated with Opacic giving Jack Tuttle an unimpeded run to the line with a back-handed flick pass to score in the corner.
They placed further pressure on the Warriors line by earning a repeat set from the kick-off and then were given a great shot to level the score when the Warriors knocked down a cut-out ball by Paia'aua and the Broncos were handed a scrum 10 metres out.
An Ashley Taylor grubber was fumbled in the Warriors in-goal to hand Brisbane a third straight set and then Paia'aua tightened the vice just a little further with another well-placed grubber into the in-goal three minutes from full-time, and four tackles later they got their reward when Jayden Nikorima speared over from a Taylor pass on the left-hand side.
Inside the last minute it gave Jayden a kick from near the sideline to level the scores but on his 18th birthday the kick missed to the left and the Warriors held on to win 34-32.
A hat-trick to Jack Gibson Medal winner Solomone Kata gave the Warriors a very handy 22-6 advantage after a frantic first half in which the video referee was called upon on on seven occasions.
The first came in just the third minute when a tap back from Nathaniel Roache appeared to go backwards for Tuimoala Lolohea to collect and score, but despite the on-field ruling of try was ruled to have been propelled forward.
Three minutes later Broncos winger Paul Byrnes was denied a try when he knocked on in trying to collect a grubber close to the tryline and then a minute later the Warriors posted first points again under controversial circumstances.
Broncos five-eighth Duncan Paia'aua was bringing the ball out inside his own half when Mason Lino and Kata combined for a tackle with the ball appearing to be stripped by Kata. Video replays showed that Paia'aua had lost control and Kata raced away to score his first of the game.
It took just a further five minutes for Kata to get his second when he collected a grubber kick from Adam Tuimavave-Gerrard and tip-toed down the sideline to cross in the corner for a 10-0 lead.
The introduction of Jayden Nikorima midway through the first half paid almost immediate dividends for the Broncos when he broke out of dummy-half and into the Warriors' back-field before drawing the fullback and putting replacement Alex Barr over under the posts.
But just as the Broncos appeared to gaining the ascendancy in the oppressive heat, the Warriors kicked again, with Lolohea being denied a try in the 32nd minute before Kata grabbed his third with Lino throwing a nice out ball to get his centre on the outside of Jordan Drew and cross wide out on the right, Lino's sideline conversion giving them a 16-6 lead.
Despite the Broncos' best efforts to stem the Warriors surge the Kiwis were in again five minutes later when blistering centre Roache burst through the Broncos defence on halfway and found Lolohea in support to race away and score under the posts and a 22-6 lead at half-time.
Warriors 34 (Solomone Kata 3, Tuimoala Lolohea, Ken Maumalo, Sam Lisone tries; Mason Lino 5 goals) def. Brisbane Broncos 32 (Paul Byrnes 2 , Alex Barr, Kodi Nikorima, Jack Tuttle, Jayden Nikorima tries; Ashley Taylor 2, Jayden Nikorima 2 goals). Half-time: 22-6 Warriors. Jack Gibson Medal: Solomone Kata (Warriors).