But just how extreme has been a matter of contention.

Now, scientists believe it could be the worst of its kind in 750 years, after making an unexpected discovery.

Researchers from the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre have identified a link between the drought, which began in the early 1970s, and snowfall at a site in East Antractica over the same period.

In research published in Nature Geoscience, they say the relationship is inverse - high snowfalls at the Law Dome site correlate with low rain in the south-west.

That is as a result of the atmospheric circulation pattern which brings dry, cool air to Australia, while sending warm, moist air to East Antarctica.

However, the high snowfall at Law Dome was unlike any other in the past 750 years, and led the researchers to believe the drought was similarly unusual.

Since the 1970s, there has been a decline of up to 20 per cent in winter rainfall in the south west, and though the cause of the drought remains unclear, others have pointed to land-use changes, ocean temperatures, air circulation changes and natural variability.

But its severity has been hard to calculate, with weather records only going back about 100 years, and the oldest tree-ring record, 350 years, from a site that has not been affected by the drought.

The researchers found that the snowfall was of a severity expected only once in every 38,000 years. Adjusting their analysis of ice cores, it still should only happen about every 5400 years.

"It also suggests... that if the mix of factors that influence (south west) rainfall over the past century reflects that of the longer term, then the recent drought... may be similarly unusual," the researchers say.

Lead researcher Tas van Ommen said the results of the study were unexpected.

"We were surprised at first, given the complexity of climate processes, to find such a direct connection between our ice core and the climate of Western Australia," he said.