The Bureau of Meteorology has today issued a La Nina alert, increasing the chance of widespread flooding during this year’s catastrophe season.

A La Nina event typically develops around 70% of the time after alert criteria are met, which is about triple the normal likelihood, the bureau says in its latest Climate Driver update.

The alert, which was upgraded from a “watch” two weeks ago, comes after the bureau yesterday warned of an average or slightly above average cyclone season in an annual outlook for the peak high-risk weather season.

“Warm waters to the north of the continent, and the sea surface temperature patterns across the tropical Pacific and Indian oceans, are driving our outlook towards more rainfall for eastern and northern Australia,” Senior Climatologist Greg Browning said.

“With an already wet landscape and above average rainfall likely, there is an increased risk of widespread flooding for eastern and northern Australia.”

The bureau says it moved to an alert due to continued cooling in the tropical Pacific Ocean and an increase in the number of climate models showing sustained La Nina conditions over summer.

The Southern Annular Mode (SAM) is expected to remain neutral for the coming week, before returning to generally positive levels from October to December.

“A positive SAM during spring typically brings wetter weather to eastern parts of Australia, but drier than average conditions for western Tasmania,” the update says.

The most recent La Nina started in September last year and continued until March this year, when major flooding hit NSW and Queensland.

La Nina events increase the chances of above-average rainfall for northern and eastern Australia during spring and summer.

The bureau also today warned that severe weather later this week could bring large hail and damaging winds in parts of eastern NSW and southeast Queensland as part of a wider system that will also affect Victoria and SA.

"Heavy rain, severe thunderstorms, snow and strong winds are expected to impact much of south-eastern Australia this week as a low pressure system moves through," Meteorologist Dean Narramore said.