Hi mate, try this...

thewindmap.com

Its a little slow to use, but is pretty good...What time of the year are you going?

Edit:
Most of those spots are for wave locations, hopefeully some of the locals on here can tell you about the flat water spots.

Here are some spots I know on the lower south coast:-

Merimbula area: Merimbula Lake (subject to tide being in), Wallagoot Lake, main beach at Merimbula (might have a bit of swell, but def. worth checking).

Batemans Bay area: North Broulee, Tomakin river mouth, Corrigan's Beach, Coila Lake (near Tuross).

Jervis Bay area: many places, especially Sanctuary Point (in Sussex Inlet)

Note: There has been quite a bit of rain on the south coast in the last few weeks. Some previously land-locked lakes have opened to the sea again. Not sure whether Coila and / or Wallagoot are now tidal.

Scan this forum for further details of these places.

PS Carry the obvious spares (eg mast base tendon, ropes). Not a lot of windsurfing shops in this region, should you break something.

Good trip, and good winds!

Mid North Coast:

South West Rocks (east of Kempsey - on Pacific Hwy) beautiful sandy bay, can camp along bay at Trial bay Gaol

Yamba river wall in NE, Iluka (opp. side of river) in S(i guess)

Nambucca river - depending on what sandbanks now doing after the floods - watch out for tides (dangerous river mouth), but great at right tides and sandbank form


Further south: Is Hawksnest, Jimmys Beach, and surrounds good flatwater?