Fairfax Media understands that Australia's Bureau of Meteorology will announce on Tuesday that the El Nino event is all but certain.

Sea-surface temperatures in the central and eastern Pacific are recording anomalies of more than 1 degree, a combination that has not previously been seen in weekly data going back to 1991, according to a bureau forecaster.

Australia's measure of El Nino thresholds is sustained warmth of sea-surface temperatures of 0.8 degrees above average in the key regions surveyed.

"You can see a warming in the eastern Pacific, which looks to be a classic [El Nino] event," said Agus Santoso, an El Nino modeller at the University of NSW's Climate Change Research centre.

Scientists are surprised that the build-up of unusual warmth in the eastern Pacific compared with the west is happening so early in the year.

"It's quite rare - this is an interesting one," Dr Santoso said.

In typical El Nino years, the usual easterly trade winds stall or even reverse in winter or later, dragging rainfall eastwards away from Australia and also south-east Asia.

Droughts tend to deepen and spread and bushfire seasons are more active than normal.

A study by the bureau of 12 strong El Nino years since 1905 found rainfall declines were most evident in winter and spring - key agricultural seasons.

The hardest hit areas cover most of NSW and parts of southern Queensland, while almost all of the eastern states have significantly reduced rain.

An El Nino event this year would be bad news for areas also suffering serious or severe rainfall deficiency.

A bureau drought report out this week identified such areas over the past 30 months to include much of inland Queensland, western Victoria and north-central NSW - some of which are already receiving federal and state aid.

However, it's the early start to the warming process that has climate scientists concerned.

". . . if it keeps going up and peaks in summer, that could potentially be a big El Nino," Dr Santoso said. With Tom Arup