New Zealand flounders are not the same species as UK flounders but similar tactics could apply. My first ever fish was in UK a flounder on earthworm for bait. Earthworm could work here particularly upstream into fresher water. Try finding some marine worms I've found a few small ragworms in Auckland. Small hooks size 2 or 4 on light trace nylon - 8 to 12 lb fished on the bottom. Most hooks available here are a bit thick in the wire for the small worms. I can send you some hooks if you want. Best of luck