A CYCLONE forming off Queensland is threatening to cause some serious damage between the Whitsundays and Fraser Island.

A low pressure system was today 650km northeast of Mackay and was expected to form into tropical cyclone Odette overnight, Bureau of Meteorology forecaster Peter Otto said.

He said it was then expected to form into a severe cyclone - category three or above - by the weekend.

"Track prediction is always difficult with cyclones but at this stage it's expected to head towards the Whitsunday coast then head down south to the Fraser coast eventually, so it could cause quite a few problems as it does so," he said.

"It should be a big system too, compared to other events we've had recently."

Earlier this month Cyclone Nelson - the first cyclone of the season - crossed north Queensland's Gulf of Carpentaria coast but caused little damage before dissipating.

Strong winds, big swells and flooding of low-lying coastal areas is expected and residents are being told to prepare.

Northern Territory residents were also put on alert, with the bureau warning a low pressure zone moving towards the coast could turn into a cyclone by Friday.

The low is responsible for the Top End's recent heavy rainfall.

In the 24 hours to 9am yesterday, Darwin received widespread downpours, with 83mm at the Botanic Gardens, 74mm at Darwin airport, 60mm at Marrara and 43mm at Royal Darwin Hospital.

East coast storms

The warnings come during a spate of wild weather which struck Australia's east coast in the past 24 hours.

In Sydney's southwest twelve cars were submerged by stormwater outside an Anglican College as storms pelted the city.

A police spokesman said no one was injured when one of the vehicles was washed into a culvert, causing water to back up and flood Menangle Road, at Menangle.

Police, ambulance rescue and the Rural Fire Service are on site.

"Traffic in Menangle is disrupted and we are asking motorists to avoid the area," a police spokesman said.

"All reports from local police are that the students from that school are not in any danger or under threat."

Departures from Sydney Airport were delayed from 2.40pm (AEDT) until 3.45pm (AEDT) because ground crews were unable work safely because of lightning strikes.

A Sydney Airport spokeswoman said crews had now returned to the tarmac and departures had resumed.

"For departures, flights were impacted on for about an hour," she said.

"We still had aircraft arriving ... but what was happening was that ground crew can't work through a thunderstorm and lightning strikes so we had aircraft sitting on the tarmac waiting to come into bay."

She said because the stoppage had occurred outside of the main peak, only minor delays should be expected and flights should soon return to schedule.

State Emergency Service (SES) spokesman Steve Delaney said dozens of SES volunteers were on standby but no calls for assistance had been received as storms moved over western Sydney.

"There's lots of water but not much associated heavy wind or large hail," Mr Delaney said.

"That's the luck of the draw, and it's been a good draw so far."

Flash flooding was recorded at Camden, Liverpool and Parramatta as the storm crossed the metropolitan area, he said.

He said small hailstones were recorded in the Campbelltown area.

Overnight, Canberra's northern suburbs were hit by a freak hailstorm which kept SES crews busy most of the morning on clean-up duties.

Hailstones as large as golf balls and 50kmh winds lashed the city for less than 30 minutes last night but left the city a metre deep in haildrifts in some places.

The storm forced the closure of the Australian National University, two high schools, government departments and many roads while it damaged the ACT Magistrates Court and ACT Policing headquarters.

Compounding matters, a low-level warning has been issued by the ESA with the bureau spokesman saying there was the chance of more thunderstorms later today.