My brother-in-law fishes the harbour each weekend and targets kingies with limited success. I was down that way a few months ago and we spent a couple of frustrating hours looking for yackas when we should have been fishing as we had purchased some quality dead bait and he had some left over frozen yakkas from a previous trip.
Kingies are predators (just like mackerel we get up my way) so while he and his mate bottom bashed I floated a dead yakka out under a balloon into a wash zone near south head. Guess who caught all the kingies?
I really don't know much about them as a species but I was watching a fishing show where the guys were casting lures around the buoys dotted around the harbour and they were killing them. If I was in Barrenjoey I would look for wash zones like Lion Is or Barrenjoey headland or some structure. If I can catch them locals should be much more helpful.