For the broadest mix of visibility and reliable weather, many divers aim for May to October in north Queensland, using hubs like Cairns and Port Douglas. Water is usually a bit cooler (often around 23°C to 26°C), so a 3mm to 5mm suit is common. November to March can still be great, with warmer water (often 27°C to 29°C), but you should plan for tropical rain, occasional trip cancellations, and seasonal stinger precautions.