The climate in Queensland can be classified as sub-tropical in the southern part and tropical in the northern part. This means that the colder months, April to October, still offer comfortably high average temperatures: even in the most southern part the coldest month (July) has an average temperature of 16°C.
The hotter months, November to March, on the other hand, can be uncomfortably hot and humid. This is especially true for the tropical, northern part, with average temperatures in the hottest month (January) as high as 31°C. Temperatures can easily rise above 35°C in this period. With a humidity of 75% and the possibility of storms and cyclones, this time of year is less attractive for visitors of northern Queensland.
In conclusion, if you have a choice, try to visit Queensland somewhere between April and October. Also, visibility is at its best in this season, and despite this being the Australian winter, water temperatures will rarely drop below 22°C. Note that June to August is also high season, with larger crowds and higher prices.