These drummer are possibly seeking cleaning by unseen hosts that are stationed on the jetty piles or the associated common kelp. I can't prove this, but again their behaviour is suggestive( they were not feeding,being outgoing tide, and they loitered persistently around the mid to upper piles,and in top L corner of 1st image you can just see a drummer's head with open mouth.That suggests the individual is more desperate than average to achieve parasite removal,based on what little is known about temperate fish cleaning.)
Adults as mixed school with Sea Sweep.
Rapid Bay Jetty
The same fish from the following observations by @dama: