El Nino may be the worst ever

The developing El Nino weather system off Australia's east coast could be the most significant on record, forecasters say.

Bureau of Meteorology acting regional director in South Australia, John Nairn, says the system is producing very warm Pacific Ocean waters over a large area and will mean lower rainfall in the east of Australia in the coming months.

"We're seeing a very large event. It's going to challenge the 1997 El Nino which is the largest we've ever seen," Mr Nairn told reporters on Tuesday.

But Mr Nairn says Western Australia and areas of South Australia can expect a wetter spring because of a competing weather system which is warming Indian Ocean waters and pushing moisture across the western half of the country.

The extent of the rainfall, the chance of a prolonged or significant drought and the likelihood of above average temperatures will be determined by which weather system wins out.

The possibility of a late season monsoon in the north of Australia is another concern heading into summer as this will impact southern temperatures.

The arrival of a monsoon late in the summer would mean areas in central Australia would heat up and stay that way for longer.

Northerly winds would push the moist air south keeping temperatures high in South Australia and Victoria.