So you were fishing 3-4km north of the town proper. Have heard of someone getting into a enormous school of Kahawai on spinners by one of these small streams. I have personally never even explored that section of the Kaikoura coast. There is simply so much good looking water down that way.
The next spot I hope to try down that way is the surf beach off Mangamaunu.. Looks to be slightly shallower and sandier then the beach you were fishing off. Interesting to hear that the fish were feeding 100m+ off shore, I have had most of my luck on short casts just behind the first wave.. Been a while since I tried to get out that far.