I read an article about 8 years ago on flounder fishing around Canterbury.
The rig the guy used was like a miniature longline made up of 6kg mono with five small size 8 or 9 trout hooks.
He had about a 2 oz sinker on the bottom end of his rig and at the top end a smaller sinker which made the rig and hooks lay flat on the floor.

Bait was live ghost shrimps collected from sandy beaches with a yabby pump. I haven't seen any in stores here, they're probably easy to make.

Fishing was done around dusk inside the estuary of the local river mouths in winter.

I've tried flounder fishing at Lake Ferry a couple of times without any luck but a friend caught one on a small piece of pipi.
Rigs we used was a small running rig with a trout hook and used pipi, crab, corn and garden worms for bait.
I'd love to find some ghost shrimps here, sounds like they're the go for bait.