At 80kg's - wave sail sizes 5.5 or 5.3 to 4.7. It rarely blows so strong that you'll need smaller than 4.7 at 80kg's and more commonly wind is around the 17-25kts in the summer for wavesailing (5.3 weather), if its less your better off with big sail and using your freerace/freestyle board at Botany Bay area. Winter will bring some southerly or westerly wind that'll get your 4.7 wet, but conditions become thick and menacing around the beaches then.
I was 78kgs and used a 5.0 and an 83lt 2007 JP RWW for 75% of the time last season and a 4.5 for the remainder. Sailed about 3-4times a month on average throughout the whole year. Though Jan and Feb 09 was more consistent and about 3 times a week

(those were the days)
Then got a 5.6 in janurary for those really light days and now its a split of 15% 5.6, 35%4.5 and 50%5.0 - though there has only been about a dozen days to sail this year to date

- I run Severne Gear Blades and S1 - but sail sizes probably the same for say an NP Fly.
Since Janurary I'm down to 65kgs and have a 77ltr board, and can get away with my 4.5 a bit more too and we had some out of the blue southerly wind come through at +30knts for a few days

....
If your into flat water/slalom and freestyle - theres a bunch of guys who sail regularly and will answere Q's in the NSW windsurfing forum for ya.