Commuter Chaos As Heavy Rain Floods Busy Road In Peak Hour

Drivers were left stranded in Sydney's northeast on Friday morning after heavy rainfall flooded Mona Vale Road.

Video shows cars on the road, which usually boasts a 90km/h speed limit, stopped as the rain continued to fall.

NSW Traffic shared an alert about 7:30 am, and the flooding was ongoing as of 8:40 am.

Firefighters are on scene working to clear a drain so the excess water can pass.

A spokesman for the transport management centre told 10 daily southbound traffic was most affected, with lane 2 closed about 1km before French's Forest Road.

The rain is also causing issues for those travelling north.

A spokesman for Fire and Rescue NSW explained after a period of no rain, it's common for leaves and waste to fill up gutters, and then be washed towards the drain en masse when the sky finally opens up -- blocking it and causing water to back up.

"There will be a few of those today I imagine," he said.

Storms are expected to cause flash flooding in many parts of Sydney today, according to the Bureau of Meteorology.

A spokeswoman for SES told 10 daily their Sutherland unit had already received 25 callouts, and there had been 13 so far in the Blue Mountains.

More to come.