Other Kaierau forwards let discipline slip as their former team mate and Ngamatapouri hooker Jarrod Wilson never missed an opportunity to remind them about mistakes.
"Whether we got sucked into the niggly stuff or overawed by it, I don't know," said Kaierau coach Denis Edwards.
"If you look at that first 20 minutes, we should have been 10-0 up.
"Some of our basic stuff we should be doing, we're not doing.
"It's the intelligence on what to do and when to do it. I don't know where we go from here."
An early leg injury to centre Osea Tarogi did not help, as Seruwalu proceeded to bust through the thinned Kaierau line with near impunity.
For Ngamatapouri coach Richard Carston, his understrength team had cleared the ledger from their large Week 1 defeat.
"They were hell bent on doing it. It wasn't too pretty at times.
"Little things - it's taking it a bit to stick to what we train, [but] we're getting better."
While still lacking in key areas, especially when it came to structure and building phases, Ngamatapouri's fourth win keeps the door slightly ajar on a possible run at the playoffs in their first Premier season.
"If we can still get through next weekend, then we get the bye and our full crew back," said Carston.
Kaierau had an early chance when winger Lafaele Malo went at the corner flag, just being taken into touch by diving fullback Carliwyne Riddles.
The home team ran back Ngamatapouri's shallow clearances to keep field position, long enough to get two penalty chances with Robinson landing the second.
Wilson caught fish with his baiting, with Ngamatapouri able to relieve pressure with penalties, and after a back-and-forth period, they got a 10m lineout and spread to Seruwalu, who bulldozed two defenders and stayed upright to go under the posts.
The visitors then went to sleep on defence as a shallow kick was claimed by Kaierau winger Taylor Kirkwood, who fed fullback Rhys Wylie coming at pace to draw the last man and send reserve Shandon Scott over in the far corner, Robinson converting from the sideline.
But if Kaierau figured Seruwalu would always just run straight, then he caught them napping with a perfectly weighted grubber kick into the in-goal with winger Jovi Maravu diving on it at the same time as Riddles for 12-10 in the 31st minute.
Flanker Corey Furness then secured another turnover by holding up the Kaierau attack in their own half, and Ngamatapouri spread to both wings with Seruwalu putting the other winger Villie Kuruyabaki into the corner right on halftime.
Kaierau then lost Yarrall for ten minutes just after the resumption for a dangerous tackle, but weathered his absence and even got some territory through the surges of lock Josh Lane, with Robinson adding another penalty for 17-13.
Vucago and Baleitavuku kept probing, but even they dropped plenty of ball, until Vucago broke a weak tackle attempt and ran through the midfield, floating the pass to Seruwalu to range wide and set Maravu up for his double.
Seemingly safe at 22-13 as the rain returned, Ngamatapouri again eased off and Kaierau scrambled hard for field position, trying to overcome their lineout issues, as visiting prop Eon Wallace was sinbinned for a professional foul stopping a rolling maul at the line.
It appeared Kaierau had bombed their opportunity when Fiatau lost the ball going off the 5m scrum, but Vucago offered another chance when he grassed the ball from Ngamatapouri's scrum and this time Fiatau made no mistake - from the new scrum he went openside to range across beside the posts.
Back to full complement with five minutes left, Ngamatapouri trapped Kaierau deep when they couldn't find touch on a relieving free kick, and Seruwalu seized another chance - taking Vucago's flat pass to bash through and drag a third tackler across for the matchwinner.
Ngamatapouri 29 (J Maravu 2, J Seruwalu 2, V Kuruyabaki tries; I Baleitavuku 2 con) bt Kaierau 20 (S Scott, F Fiatau tries; E Robinson 2 pen, 2 con). HT: 17-10.