A very "fast-moving" band of rain has brought an unexpected large amount of rainfall, submerging Sydney's CBD in panic-gripping flash floods on Wednesday morning.

Almost 30 millimetres of rain fell in just half an hour, according to the Bureau of Meteorology. But it was enough to submerge some areas in floods, including roads around Mascot, Paddington, Marrickville, Moore Park, Bondi, Waterloo and the CBD.

Not only did the rainfall caused traffic in major roads, it also forced railway lines to suspend operations.

Although residents and commuters had expected isolated showers, what occurred was much heavier than expected.

The band of rain started to pour in the Illawarra and then moved quickly north towards Sydney just in time for the morning peak rush hour.

In just half an hour, it dropped 20mm amount of rainfall at Sydney Airport and 14.4mm at Terrey Hills.

"The models started to show it early this morning, we saw a change moving up the coast and it seemed to target Sydney with a lot of the moisture which has been coming in off the waters," Ben Domansino from the Weatherzone told ABC.

"The good news is that it'll push through quite quickly."

"It's expected to ease, it's already easing through southern parts of Sydney, and as it pushes through the upper north shore we'll see most of Sydney easing back, to those scattered showers, so it's unfortunate that it's come at peak hour," he said.

More rain however has been forecast for Thursday.