The weather bureau announced a severe thunderstorm warning with hail and damaging winds over New South Wales. True enough, storms blacked out 150 traffic lights across Sydney and closed parts of a major rail line.

A fallen tree got stuck in the overhead wiring at Denistone, suspending train services between Rhodes and Epping on the Northern Line.

Weatherzone meteorologist Alex Zadnik said a cool, dense outflow of air from merging thunderstorms brought damaging wind gusts of between 60-80km/h to Sydney, along with torrential rains.

Sydney was hit Tuesday by thunderstorms so severe, that all take-offs and landings were halted for 20 minutes, reports the Sydney Morning Herald.

A record of 16mm was made in just 10 minutes at the airport. This is almost one-quarter of the monthly average and is more than enough to cause flash flooding, reported the Herald.

Earlier in the day, a thunderstorm produced a tornado near Bombala, while wind gusts of close to 100km/h were recorded at Tibooburra, north-west of New South Wales.

Weather Bureau senior forecaster Peter Zmijewski said an "almost semi-permanent" trough had developed over inland Sydney, causing the evening storms.

"We're warning about a patch of severe weather, large hail stones, large flooding and damaging winds," he said. "In other words, everything."

Wednesday evening would see thunderstorms again, but by Thursday, the trough would most likely contract, isolating the storms to the north-east of the state, Zmijewski said.

A spokeswoman for the State Emergency Service, Becky Gollings, told the Herald the state has been lucky as no major damages have been reported so far.