A man has died after being sucked into a storm water drain in Lucas Heights during flooding rains which have blanketed Sydney.

Morley Ave, Roseberry was turned into a river by the heavy rain. (Twitter:Cam O'Neill)

Ambulance Service of NSW paramedics responded to reports a man was stuck in a stormwater drain on New Illawarra Road, Lucas Heights.

An ambulance spokesman said the man was dead when paramedics arrived at about 3:15pm.

Police say the coroner and WorkCover NSW are investigating the circumstances that led to the man's death.

ABC News report police are attempting to recover the man's body from the flooded drain.

NSW State Emergency Service (SES) has successfully undertaken six flood rescues as heavy rain causes flash flooding across Sydney.

Paramedics from have treated a 19 year old man for electric shock after a lightning strike in Mascot.

Powerful rain from a low pressure trough just off the central part of the coast dumped up to 70mm on parts of Sydney south of Little Bay on Monday.

Cronulla experienced rainfall over 70mm which fell within one hour before 3:00PM.

The rain caused flash flooding across greater Sydney, closing roads and causing water mains to burst.

Roads were closed by flooding in Roseberry, Zetland, Heathcote, Paddington, Sutherland and Menai.

Phil Campbell from the SES says there are reports of cars floating around the Bunnings car park at Bankstown and also localised flash flooding in other parts of the city.

"We've had reports of people who've been trapped in their cars so we send out our specialist flood rescue team," he says.

"But the lesson, of course, is don't go in (to floods) in the first place."

"We're seeing these very fast-rising waters. So if you do encounter that sort of water the best thing to do is to find an alternate route home."

Cronulla South Bowling Club recorded 55 mm in 30 minutes to 240 pm, and 63 mm in 1 hour to 249 pm was recorded at Audley (Royal National Park).