Saracens had pace to burn but, similar to Taranaki Development XV the week before, made key errors in handling and misdirected passes, especially out wide with the tryline beckoning.
Whanganui, in contrast, tried more short passes and inside balls on second and third phases after strong carries and found half-gaps opening up.
The reunited midfield pairing of Timoci (Jim) Seruwalu and Kameli Kuruyabaki were both able to step through scrambling cover defenders who had been pulled in by that work, scoring on the edges, while lock-turned-flanker Ben Whale really stepped up by pumping through tacklers for the visitors' first two tries.
Whale also had a strong lineout performance, as did fellow flanker Mason Johnson and reserve Matt Ashworth. By the end of the game, Saracens were taking taps on every penalty because they no longer trusted they could win lineouts or scrums.
The latter was thanks to reserve prop Gabriel Hakaraia taking over up front when he came on, following on from hooker Roman Tutauha and prop Renato Tikoisolomone, who also did some nice short offloads, especially to his cousin and hard-working winger Alekesio Vakarorogo.
It was Vakarorogo who galvanised tiring limbs in his team by snatching an intercept to run 80m and score in the second half and pull Whanganui out to a 33-10 lead after it looked like Saracens were going to get over the other way following multiple hard carries.
Skipper Dane Whale had more time and space to use his kicking game effectively, especially with a nifty grubber for the team's speedster in reserve winger Peceli Malanicagi to regather and score – Whale and halfback Kahl Elers-Green having come back on to run the cutter after Rogers-Holden's and Tremayne's departures.
Facing some of his Taradale clubmates, winger Ezra Malo took the kicking tee and coolly slotted five from seven conversions, putting further scoreboard pressure on the home side.
The home side did rally with two late tries, but Whanganui's line held up to a lot more of a battering as the scoreboard ticked away, and then they trapped the home side in their pocket for slippery reserve winger Tiari Mumby to sneak off the ruck and score on fulltime.
"Last week, we were off the mark with people who came on, and I think this week they were closer to the mark," said Hamlin.
"They're getting used to the pace of the game again at this level.
"There's still lots of stuff to work on. Defensively, one-on-one tackles were missed that we can't afford, and by the looks of it, a lot of it was shape stuff.
"Guys were hinging in when they should have just been staying connected and occupying a space, and then we could have shut stuff down, but we gave them avenues to attack down.
"If we can stop that stuff, we'll go a long way towards controlling the pace of the game."
There were nervous times when Saracens scored immediately after Malanicagi's try to reduce the gap to 13 with time remaining, invoking memories of the week before when Taranaki's Development XV snatched the win in the last minute.
But this time, the composure was there, Whanganui having enough fuel in the tank to finish strongly.
"We're seeing growth this week around closing the game out, doing all the little things right. It wasn't perfect, but it was better," said Hamlin.
"Someone like Matt Ashworth comes along and really contributes in that lineout space, and that's going to be really critical for the next couple of weeks."
Given his father was a Whanganui captain and his older brother now holds the role, it is no surprise to see Ben Whale stepping up as a leader in the pack.
"He's doing lots of stuff. Semi [Vodosese]'s not a big communicator but he's a hard worker, and Jamie [Hughes]'s not a big communicator but he's a real hard worker," said Hamlin.
"So we need someone, an energy person who says the right things because it then puts people in the right places.
"You see Kameli and Jim are actually attacking spaces, not running into bodies, little things of that nature."
Steelform Whanganui 45 (Ben Whale 2, Kameli Kuruyabaki, Jim Seruwalu, Alekesio Vakarorogo, Peceli Malanicagi, Tiari Mumby tries; Ezra Malo 5 con) bt Hawke's Bay Saracens 27 (Majella Tufuga, Josh McIntyre, Zach Simpson, Alex Philip, Iakopo Mapu tries; Jayden Rihia con). HT: 21-10.
