Hi Si,Went to Kalbarri for exactly the same reason as you in march 07.The river is great for learning,maybe a bit narrow but the water is clean and warm.
It was taking my grandson 45 secs from one side to the other in about 15 knots of wind(plaining).Lots of shallow water in between the jetties for teaching.
Look up Kalbarri on google earth for the big picture.For me the river was just fine as the ocean in that area is very unforgivingfor all but the pros.
ENJOY.

Hi

Launch in the river close to the boat hire on the SW and you can travel quite happily from one side of the river to the other.

Seabreeze usually kicks in about 1030am-1100am and drops out around 300pm (in the river because it swings slightly).

On an easterly its good to launch at the car park up river of the boat hire and u can plain in the shallows parallel to the shore.

I'll be up there for a holiday with my girlfriend from the 11th so you will see other windsurfers around :)