Metro's best chances came when centre John Iro made a good run, and then winger James Dorgan snatched an intercept and raced deep into WHS's 22m zone, yet on both occasions the ball was lost a few phases later.
However, after a kicking duel across the field, Puki-Kaa probed forward and then second-five Daniel Kaikua chipped ahead and showed cool timing to judge the bounce and score in the corner two minutes before halftime.
Despite 5-5, WHS were confident as they now took the wind advantage and would have the better of territory for most of the second half as Metro made some key fumbles.
Standout WHS first-five Mitchell Millar was creating opportunities as his team would quickly turnover any lost possession, and Metro finally broke on their goal line as the ball was spread to unmarked winger Aiden Hunter.
Millar tried to make the game safe with a couple of long-range penalties that narrowly missed, but then it seemed WHS had scored the matchwinner with five minutes to go as again they spread from sideline to sideline and winger Magnes McLaughlin dove through the last tackler to score.
However, Metro would not go away as they swarmed the kickoff to regather and winger Shayde Chadwick-Waitokia beat three defenders to score in the far corner, with the sideline conversion just missing.
WHS then kicked out on the full to leave a scrum on halfway with time virtually up, but Metro could not hang onto the ball in what was the story of their afternoon.
Nonetheless, coaches Carl Gibson and Darryl Malcolm can be justly proud of the efforts of their new look team in its debut 2016 season, a squad which was created from recent school leavers and young men who had otherwise given up the game.
Meanwhile, WHS have a date at Arena Manawatu this Saturday at 1pm to take on Massey White and decide the 2016 Colts title.
Both Whanganui teams linked arms in the middle of Spriggens Park at the conclusion of Saturday's semifinal, leaving no doubt that WHS will go for glory with Metro's best wishes.
Wanganui High School 1st XV 15 (A Boult, A Hunter, M McLaughlin tries) bt Wanganui Metro 10 (D Kaikua, S Chadwick-Waitokia tries). HT: 5-5.