The victory mirrors Kaierau's defeat of Pirates in their first game last year, while closing the gap on the points table so McCarthy's Ruapehu and Taihape both have an outside chance at sneaking up and claiming the Paul Mitchell Cup as Round one winners this coming weekend.
This would require Pirates to have a second bad week in a row against Dave Hoskin Carriers Marist, whose hard run at the back end of the round continued with a 29-8 loss to Waverley Harvesting Border at Spriggens Park.
Border wrapped up their Round 1 with a four tries to one win, as winger Sikeli Ulukuta had another profitable day with 14 more points for his personal tally.
Down at Marton Park, it was another tight tussle for the Grand Hotel Challenge Shield but Harvey Round Motors Ratana maintained their tenuous grip, beating Speirs Food Marton 23-19.
Perhaps learning from their 13-0 defeat on the same ground in 2013, Ratana did not ignore kickable points when they were on offer as first-five Caleb Maraku landed three penalties to go with conversions of second-half tries by Te Tua Kemp and Jamie Hughes.
That goal kicking would prove the difference as Marton got three five-pointers through Tyler Rogers-Holden, Meni Faasau and Mundsan Ah Chong.
Ratana remained poised as the only team outside the Top 4 who remain in semifinal contention.
In Senior rugby, Marist Celtic picked up a 25-8 win over Border.
Defending champions Power Farming (PN) Hunterville got back on track with clear cut 41-17 victory against Marist Buffalo's, while Taihape made their first defence of the Stihl Shop Wanganui Challenge Shield with a 28-10 local derby win over Utiku Old Boys.