Feilding scored in the opening minutes from a scrum attack and offload to put speedy winger Moape Rokosuka over in the corner, but missed both the conversion and a penalty to extend their advantage.
A series of penalties let Collegiate's forwards work into position and flanker Tom Pease drove through a gap to score the equaliser, although first-five Harry Godfrey's conversion was ruled by referee Ben Lourie to have missed despite each school's assigned touch judge raising his flag - given neither was an official AR it is the referee who has the say.
Feilding regained the lead 12-5 after a superb pack drive from a penalty lineout for hooker Michael Strydom to score at the back, but Collegiate immediately replied in-kind with the visitors offside at the restart – taking the lineout and driving prop Leon Allan across.
Trailing by two points at halftime after a helter-skelter final few minutes before the break, with attacking kicks and fumbles from both teams, Collegiate had by now absorbed Feilding's big hits on the carry and still kept coming.
They started to make the advantage line with quick recycles, setting up No 8 Josefa Namosimalua to dive over beside the posts for an easy Godfrey conversion.
After a tense 20 minutes with both sides spilling passes, Feilding levelled 17-17 through flanker AJ Malili-Malo-Lauano scoring after two phases off a 5m scrum win, but that man Godfrey would not be denied.
With five minutes left, the captain scored a fantastic try by beating three defenders, and Collegiate immediately got back on attack from the restart to earn a penalty, which Godfrey slotted to put his team clear.
Feilding pressed desperately for the final couple of minutes, but good Collegiate defence forced them into more errors which proved the bugbear of their afternoon.
Collegiate will now travel to the neutral venue of Owen Delany Park in Taupō for their semifinal with the North Region's second qualifier St Peter's Cambridge, who claimed the spot on points differential after their last game with Level 3-locked Wesley College was cancelled and ruled a draw.
At the same time, Feilding High will take on St Paul's Collegiate, who qualified top of North Region with a 3-1 record, and the winners will return to Taupo for the final next Saturday.
Whanganui Collegiate 25 (Tom Pease, Leo Allan, Josefa Namosimalua, Harry Godfrey tries; Godfrey pen, con) bt Feilding High School 17 (Moape Rokosuka, Michael Strydom, AJ Malili-Malo-Lauano tries; Jonty Stewart con). HT: 12-10 Feilding.
