You can travel to Cairns any time of the year. Cairns is full of tourists all year around. But the best time to visit Cairns is during our winter (June to end of August). The weather is perfect, blue sky every day and very comfortable temperatures (average 25C). Also there is no stingers (jellyfish) in the water so safe to swim anywhere around Cairns. Rainfall is quite low at this time of the year so usually no thunderstorm or torrential rain.
In November starts our wet season and it goes until end of March. During this time of the year we usually get high rainfall, thunderstorms and possible cyclones. Cairns hasn't had a cyclone for many year but the ones that passes near by can bring lots of rain to Cairns. Even though the wet season starts in November usually the heavy rain doesn’t start until January. Jellyfish comes back to Queensland cost in November so you are allowed to swim only inside the nets on the beaches or anywhere if you are wearing a stinger suit. If you are travelling to one of the islands or the reef the tour operator will highly recommend you to wear a stinger suit for protection.