This year's rainy season in Australia is linked to La Niña, a phenomenon which occurs when water in the equatorial region of the Pacific is cooler than normal, resulting in a path of heavy precipitation. 

However, the answer may not be that simple. Many people, including climatologists, believe that the unusual intensity of these rain falls have been contributed to by global warming.

The exceptionally heavy rain in Australia is being caused by this year's particularly strong La Nina cycle. During La Nina cycles areas like Australia experience heavy rains, whereas an El Nino cycle can cause drought in those same areas. In a nutshell, La Nina is a cooler and than "normal."