sydney, melbourne, cairns

thinking of going to sydney, cairns, melbourne from march 25 - april 6 this year. is the weather likely warm (75F) for the beaches? how unpredictable is temp in cairns? love the ocean/great barrier reef. worry that the storms will continue until the time i travel and we'll be stuck in hotel not being able to do anything.

Cairns will be warm. Melbourne will be cold, and Sydney may be warm enough for someone from a colder climate to swim.
Most storms are not continuous - it rarely rains all the time in the tropics, more likely a little each day.
However, you have posted this on the wrong branch. This one is for the Pacific islands. There is another one for Australia and New Zealand, where you should copy this post.