Hi Neil, theres a few shallow water weed fins developed in recent years for spots like primbee, some have made their own fins (eg keef, if you search on here you'll find some good threads on the subject, try also searching on lake George fins and budgewoi fins). The tectonics speed demon fins go well in the shallows, probably a board that size would need a 23, depending on your sail size. I use 19 with a 6.2,and 21 with a 7.0m sail, smaller boards than yours. There's other fin brands out there, hopefully you'll get some more views of what to go for that board

Fins used at Primbee tend to get their own custom mods by the shells in the sand. G10 fins are a much kinder to your bank balance and a little more durable than carbon

Hi Neil,
Welcome to our playground. Whatever fin you land on, make sure to talk to some locals before you take it out. Based on the water level on that day (it varies with not only tide but also rainfall and wind speed) they'll give you an idea of where it is safe to sail with your given fin depth. You can then build on this knowledge over time.
When in doubt, play follow the leader!!
cheers,
Det

Check out Atomic and Tribal brand weed fins.

Both are Australian designed fins.

www.atomicwindsurf.com/fins

www.tribalwindsurfing.com/en/13-fins

i have plenty of shallow water fins for sale give me a call 0403348278