Two children, a three-year-old girl and a five-year-old boy were in the car at the time of the crash.

Earlier on Friday, North Sydney was bracing for dangerous thunderstorms and at one point the area received 5.2 millimetres of rain in ten minutes, before the worst of it passed. The suburb received 23.4mm of rain between 9am and 6pm on Friday. That was exceeded only by Horsley Park in the west, which received 27mm.

The storm came as Sydney was still recovering from severe thunderstorms which moved across the city on Thursday evening.

More than 40,000 properties were left without power and a number of major roads were flooded during that storm. The Bureau of Meteorology recorded 40 millimetres of rain in just 30 minutes at Auburn during its peak.

The State Emergency Service received 1030 calls in the 24 hours leading up to 6pm on Friday from across the state, attending more jobs in western Sydney than anywhere else in the city.

"The message we are trying to get out today is to not drive through floodwater; there are more heavy rains forecast so we want people to avoid unnecessary travel," NSW SES media officer Phil Schafer said, having conducted at least seven flood rescues after motorists became trapped in cars.

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Elsewhere in the country, Queensland's far north was preparing ahead of the arrival of Tropical Cyclone Owen, but it was taking longer than expected to arrive, leaving residents with a nervous wait.

Owen has been tracking over the Gulf of Carpentaria for the past few days, re-intensifying on Thursday into a category 3. The cyclone is expected to be upgraded to a category 4 on Friday night or on Saturday morning as it makes landfall.

"The U-turn has taken a little bit slower and is a little bit further to the coast than we were expecting yesterday," the Bureau of Meteorology's Laura Boekel said on Friday.

"Systems in this area are notoriously erratic and move quite quickly and the situation can move quite quickly as well."

Victorians are also being told to watch for more flooding as the state gets deluged with record rains.

Severe thunderstorms and flash flood warning were issued for Seymour, Ballarat and Melbourne.

More than one hundred people needed rescuing from the Hume Freeway south of Wangaratta, including 17 by helicopter, due to floodwater.

All up, 400-500 vehicles were impacted by floodwaters on the Hume Freeway, according to the alert.

More than a month's worth of rain has fallen across parts of Victoria. The wet weather is expected to continue over the weekend.

With Fleta Page and AAP

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