Sailed in Darwin from 1990 to 1997. Used to sail out of the little park that is called Rapid Creek Beach , Casuarina Drive, Rapid Creek on Google maps. ( No Croc's around then )
Wet season westerlies would get up to half mast on shore waves ( Wave gear ). Dry Season would be off shore South Easterlies trade winds (Slalom gear ) which normally would turn off by early afternoon. Also sailed at East Point In front of Lake Alexander and inside of the sandbar at Cullen Bay.
As Wildrover said, go see Geoff at Fun Supply ( who I used to sail with) for advice. Although when I went back to visit in 2009 all the guys had gone over to kiting and very few windsurfers left.
In any case, if its windy ,go for a drive, Darwin is a small place.
Ps Make sure you wear a stinger suit as you are more likely to die from a Box Jelly Fish sting than be taken by a Croc