Australia remains on course for its first El Nino in more than four years, a weather event that typically brings a drier-than-average winter and spring to much of southern Australia including NSW, and active bushfire seasons by summer.

Warm sub-seawater temperatures – in places as much as 6 degrees above normal - continue to spread eastwards in the tropical Pacific, a key precursor to an El Nino forming, the Bureau of Meteorology said.

The bureau rates the likelihood of an El Nino at more than 70 per cent, with confirmation possible in the next month or so, manager of the bureau’s climate prediction services, Andrew Watkins said.

“We’re waiting to see the response from the atmosphere,” Dr Watkins said, adding that that signal might take months to become clear.

Normally, trade winds blow from east to west along the equatorial Pacific. As warm waters spread eastwards, as they are now, sea temperature differences narrow, causing those winds to weaken and clouds to build up near the dateline.