The Iluka Bluff has weed growing there all of the time. You have to search a bit for it amongst the cabbage. The most reliable area I've found is about the middle of the headland roughly in line with the lookout. You'll need to be there near low tide. It does get fairly heavily picked over at times but it grows pretty fast, so if you're the first one there with a low tide around dawn you should have no trouble getting enough for a couple of river sessions
Cheers Freeeedom