It's evidently a combination of factors Jack.
There's a weather phenomenon called 'La Nina' which turns up in an irregular cycle - usually with a 3 or four year time frame- and it brings moisture laden air from the Pacific Ocean over the Northern sections of Australia.
This year there was some sort of extra interaction with the normal monsoons of the tropics and the air was carried further inland and further south than usual. (with a vengeance - we've had floods here today in Victoria, right in the south of the continent.)
As well, the water temperature in the ocean is approx half a degree Celsius warmer. I'm not sure of the time frame for that rise but a small increase in temperature causes a big increase in the amount of evaporation.
All that compounded to bring the astonishing, tragic, and devastating results.