It follows yesterday's forecast of a return to El Nino conditions later this year.

The Bureau says that when the waters off Western Australia heat up, evaporation pumps moisture into winds, creating north-western cloud belts that blow from the Kimberley, over Central Australia and into NSW and Victoria, where it falls as rain.

"You clearly see the clouds on satellite images, looking like a slash of paint across Australia," Andrew Watkins, a senior climatologist with the National Climate Centre, said yesterday.

When the eastern Indian Ocean is warmer than the sea off Africa, climatologists give a negative balance rating to a system dubbed the Indian Ocean dipole.

That is good news for NSW and Victorian farmers seeking rain.

But when the reverse happens, and the eastern Indian Ocean becomes relatively cooler, evaporation off WA falters and the dipole is rated as positive.

Yesterday the eastern Indian Ocean's cooling forced climatologists to rate the dipole balance as plus .6.

"Plus .4 is where we start thinking something is going on," said Dr Watkins, adding it was "starting to get significant.

"Over the past month we have been starting to see the imbalance rise."

Such imbalances often signalled drier winters and springs in south-eastern Australia. "Whether [the trend] will be sustained, we don't know," Dr Watkins said.