Oz clouds

Extreme weather patterns have hit Australia with tropical cyclones, snow and heatwaves this month - but the weirdness is set to continue with an active monsoonal trough over north Queensland and a storm front heading for Sydney this evening.

Thunderstorms and heavy rain lashed Sydney, the NSW south coast and parts of the state's inland last night.

The storm dumped more than 100mm of rain over one hour in what the Bureau of Meteorology (BoM) described as a "one in 20-year event".

The continuing La Nina pattern has brought above-average rains and cooler temperatures this summer, weather experts say.

But that pattern has been twisted to extremes over eastern Australia, with cyclone activity and flooding over Rockhampton and Mackay in Queensland, as well as a cold front in Tasmania that brought snow on the weekend.

West Australians meanwhile are sweltering under a heatwave that saw temperatures soar over 35 degrees in recent days.

"Thunderstorms are pretty common at this time of the year, but we're seeing all sorts of other things too," the Weather Channel's Dick Whitaker told Today.

"We've got a very active monsoonal trough at the moment over the north which is giving us all that heavy rain, and very clear conditions over in Western Australia, so there's a lot of contrast."

Mr Whitaker said the trough spanning the Northern Territory and northern parts of Queensland would continue to deliver heavy rain, while a low pressure trough heading for Victoria may spawn thunderstorms in coming days.

And the lightning and thunder that hit Sydney and other parts of NSW last night - prompting about 100 requests for assistance from the State Emergency Service - is apparently only a preview for more severe weather this afternoon.

BoM senior forecaster Chris Webb said the predicted storm front could weaken by the time it reached Sydney, but there is a chance it will resemble Tuesday's deluge.

"The Illawarra area, parts of the south coast, also Sydney metropolitan and parts of the Hunter are the prime areas for some severe weather this afternoon," Mr Webb said.

"I think it's a real possibility."