The rainy season is most problematic round the equator. The Great Barrier Reef and the Daintree get the most rain in January and February, when it rains about half the days in the month, most often in the afternoons.

Sydney doesn't really have a rainy season, it's not really monsoonal country that far down.

You can still dive on the reef in the rainy season if that's what this question is about - the visibility isn't as good but there are still some good, clear days. It's humid though.