Wairarapa Bush attacked after the break and after losing the ball over the line following 10 minutes of pounding, they quickly forced the turnover and centre Robbie Anderson crashed over in the corner.
Having played with little territory, Wanganui finally began to get some traction, helped when reserve Steelie Koro, who had rushed back from a work appointment 30 minutes after the game started, got on to the field - much as he would for the Wanganui seniors later in the day.
The game changer came in the 57th minute as Wairarapa Bush miscued a clearing kick, and nuggety winger Tom Symes came forward to make a brilliant catch and beat his man in the same motion, diving over to give Wanganui back the lead, 15-11.
Feeling it now, Wanganui got back on attack and trapped Wairarapa Bush in-goal for a 5m scrum.
A few phases later, the Wanganui senior's wider-squad prop Tawhiro Gupwell bounced out of two tacklers to crash in beside the posts, although he stayed down after a nasty blow to the face.
Play was fairly scrappy from both sides in the final 12 minutes, but Wanganui were home and hosed.
"It's pleasing really, in the tough conditions," said Williams.
"The lead-in was not great with a bye and the default. At Thursday training we were reminding them how to play it.
"We eventually got into it - we've got a lot of firepower in our outside backs.
"When [Koro] turned up, he added a lot more direction."
Williams said he was proud the team had been as cohesive as they were, given they came from so many different local clubs.
Wanganui Development XV 22 (Kane Tamou, Tom Symes, Tawhio Gupwell tries; Te Tua Kemp pen, 2 con) beat Wairapapa Bush Development XV 11 (Robbie Anderson try; Himiona Haira 2 pen). HT: 10-6.