Waikato turned in a disjointed performance and compounded their woes with an indisciplined display that let Waenga punch over morale sapping penalties in the first half.
Waenga opened the scoring with a penalty that Waikato conceded under pressure in defence and then set his backline alight to for Arnold to score the first try after chiming into a sweeping move.
The first five-eighth converted and then added another two penalties, one from long range, to seal a 16-3 lead at halftime. Waikato's points came from a penalty taken by first five-eighth Trent Renata.
An early second half penalty by Waenga had Waikato back-pedalling for the rest of the match and Arnold quickly got his second try after lively wing Lelia Masaga barged past two feeble tackles.
Waenga then glided through the midfield and his break set up substitute hooker John Paraenga for his side's third try.
Waikato grabbed a try through lock Toby Lynn against the run of play only to see Arnold unlock their defence with his run late in the match, his offload giving substitute halfback Taniela Moa the simplest of tasks to plant down his side's fourth and bonus point try.
The victory sees Bay of Plenty top the standings with 14 well clear of closest pursuers Canterbury and Auckland who have six points each.
- NZPA