Can you be more specific? there is 2,000+ miles of coastline between the two cities and more diving than you could do in a lifetime...
A must do is the HMAS Brisbane out of mooloolaba about 1hr north of brisbane sunk in 28M, weather can be a bit shitty though so best plan a few days in the area,
Sydney has fantastic diving of all levels from 5m bimbles to 75m+ Wrecks
Jervis bay 3 hrs south of sydney has fantastic liveaboards and excellent diving
water round sydney is from 16 in the winter to 23 in the summer.
cant help south of there although I hear the water's cold 'Brrr'
Most of the dive shops seem pretty good as long as you avoid pro-dive although they do seem to have the shiniest boats