A major El Nino event, which is likely to lead to prolonged drier, hotter conditions across much of eastern Australia, has been officially declared by the Bureau of Meteorology (BoM).

According to the Bureau, an El Nino occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become substantially warmer than average, causing a shift in atmospheric circulation.

The impacts of the event are likely to be felt until autumn next year and 17 of the last 26 have resulted in widespread drought.

"El Nino events are not always associated with drought over Australia, but they are often associated with drought, with the main impacts over eastern Australia," the BoM's Dr Lynette Bettio said.

Rural news in your inbox Subscribe to get ABC News delivered to your email, including top rural coverage, plus the day's top news and analysis and alerts on major breaking stories.

Other impacts include:

Drier than average winter-spring

Drier than average winter-spring Increased daytime temperatures in southern Australia from winter onwards

Increased daytime temperatures in southern Australia from winter onwards Increased risk of frosts in winter due to dry soils and less cloud

Increased risk of frosts in winter due to dry soils and less cloud Reduced snow cover and shorter season

Reduced snow cover and shorter season Early and more extreme fire season

Early and more extreme fire season Reduced chance of widespread flooding

However, the weather event is not all negative with tropical cyclones less frequent during El Nino.

"We would expect the tropical cyclone season would be below average," Dr David Jones, who is also from the BoM, said.

"But it only takes one bad cyclone, so we want people to take care, but the odds are reduced.

The Bureau of Meteorology explains how El Nino is defined. ( Supplied: Bureau of Meteorology )

"Every El Nino is different and we know that some years like 1972, 1982 and 1994 really fit the stereotype strongly, severe drought, very hot daytime temperatures, bushfire activity and so on.

"But not every El Nino event follows that pattern.

"1997 for example was a very strong El Nino event [that] wasn't too severe in the Australian region."

This declaration is likely to provide little comfort for graziers, particularly in parts of NSW and Queensland, who have already been hit hard by drought.

James Walker from Camden Park, near Longreach in Queensland, has destocked his property because there is no grass for livestock to eat.

"This year we've written off production," Mr Walker said.

He said the El Nino effectively meant he would not be relying on any regular rainfall.

"With an El Nino you've got opportunities of isolated thunderstorms and showers.

"If you can get a few of them you can get a bit of a season and some opportunity, but in terms of production you can't really plan for production when you've got an El Nino.

"Particularly with an engaged El Nino the likelihood [of rain] is very low.

"I just think the wheels have to spin faster, we have to make better decisions, plan better and also mitigate with the overheads that are plaguing on businesses out here.

"A lot of people have already made the decision that it wasn't going great this year.

"I've got a fair bit of conviction in adapting and I treat the situation as a challenge and a challenge we can overcome."

The last event was in 2010 when large parts of the country were affected by severe drought.

The Bureau said it had a "near miss" with declaring an event last year, with some indications showing sea surface temperatures were at El Nino levels.

But these signs did not all occur at once.