But then the floodgates opened again as the Samoans, who had a bigger bench both in terms of numbers and size, began punching high speed holes in the tired defence, with prop Jeremiah Tuiatua and fullback Fritz Rayasi nearly unstoppable.
Their team ultimately scored nine tries but the scoreline stayed under 50 as Wellington's three goal kickers could only add one conversion between them, even missing drop kicks straight in front.
It was looking like a long 70 minutes when first-five Lemafa Metuli-Tuluauau found a half gap to dive over in the fourth minute, then second-five Eli Moata'a made another break and from the breakdown at the tryline, Tuiatua cruised under the posts with Rayasi then adding their only extras of the game.
The hits kept coming as lock Akira Ieremia also jogged over from a tryline ruck five minutes later after Wellington surged out of their half.
But Wanganui tightened up and scored a good 60m try as fullback Jack O'Leary ran through a gap out wide and then stepped back towards the posts, finding centre Te Wanahi Rowe to cross in the 23rd minute.
With Wanganui getting territory with the boot as the Samoans got testy with the official, O'Leary was able to add back-to-back penalties just after halftime.
But then an lineout throw not going straight left Wanganui under pressure and when the backline defence rushed up too early, Metuli-Tuluauau dashed straight through to score.
A backline spread saw Rayasi fend off the cover defence to score in the corner, and then Wanganui panicked with some wild passes in their own half after winning a turnover, with Metuli-Tuluauau chasing the loose ball to get his hat trick.
The ball was now being flung everywhere as Wellington winger Herman Seumanufagi was on the end of the chain to score, and then reserve Shaquille Fiso just palmed off defenders to go under the posts.
Wanganui had a rare chance with the ball and tried a chip and chase, but the Samoans just dashed back from their 22m and reserve Steven Va'a eventually scored under the sticks on fulltime.
Wellington Samoa U18 47 (L Metuli-Tuluauau 3, J Tuiatua, A Ieremia, F Rayasi, H Seumanufagi, S Fiso, S Va'a tries; Rayasi con) bt Wanganui U18 (T Rowe try; J O'Leary 2 pen, con). HT: 17-7.
Meanwhile, the New Zealand Heartland XV were ultimately unsuccessful at the 2018 Jock Hobbs National Under 19 tournament in Taupo on Saturday.
In the wooden spoon match at Owen Delany Park, Heartland were beaten 24-12 by Manawatu.
The team did well to come back after being down 17-0 at halftime, but ultimately in wasn't enough.
It is the first time since 2014 that Heartland has not picked up a victory from their three tournament games, after they were hammered by Waikato 67-5 the previous Sunday, then lost 38-35 in the last minute to Northland on Wednesday.
Whanganui flanker Jason Myers and midfielder Desmond Tyrell were in the squad.
Myers could now return to join the Air Chathams Wanganui Under 18's to face Wairarapa-Bush U18 this weekend.
