The Bureau of Meteorology (BOM) has confirmed there is now a full-blown La Nina in the Pacific Ocean, which is positive for above average rain down the east coast, especially in Queensland and northern NSW.

On the other side of the continent, there is a strongly negative Indian Ocean Dipole (IOD), which is consistent with above average rainfall for Western Australia, parts of South Australia and north-west Victoria.

WA needs above-average rainfall to eke out an average crop, while some finishing October rain on the east coast would have farmers set up for a bumper season.

“We haven’t got our new rainfall outlook out yet, but it would appear that the August figures we issued are on the conservative side,” the BOM’s National Climate Centre senior climatologist, Robyn Duell, said.

“While they do not always lead to above-average rainfall, both events are associated with rainfall above normal during the second half of the year.”

Ms Duell said the IOD event was likely to influence weather during the spring before decaying, while La Ninas generally lasted into the summer.

She said the IOD event would begin to decay toward the start of November, when the Australian monsoon season began.

The majority of computer models used by the BOM indicate the La Nina will persist into at least early 2011.

Farmers in south-eastern Australia will be pleased with the prospects, given rain has eased off since floods at the beginning of September.

There has been little meaningful rain in northern Victoria or South Australia since, and little on the immediate horizon, as a slow moving high pressure ridge means a cool, dry pattern with south-westerly breezes has dominated.

The dry has been welcomed by farmers looking to get onto paddocks, but finishing rain will be needed to ensure crops live up to potential.

However, Ms Duell said the recent lack of rain did not mean the year was likely to go the way of 2007, when a La Nina did not stop south-eastern Australia suffering from an extremely dry spring.

“It’s important to differentiate between weather patterns and climate and this period of high pressure is just part of a weather pattern – it needs to go on for at least a month before we start thinking in terms of a change to the overall climate pattern,” she said.