Flash floods have impacted many towns and city roads, the SES said on Tuesday afternoon.
Rescue crews have attended to at least 15 flood rescues across Sydney and the Central Coast with 450 call-outs statewide.
"We are imploring you with 15 flood rescues in inner Sydney and up at the Central Coast, that's too many... Please don't drive through floodwater," the SES said.
Flash flooding inundated various Sydney suburbs with Marrickville in the inner west recording at least 110mm.
Wyong in the Central Coast was battered by 80mm in 45 minutes only.
The Bureau of Meteorology said the immediate threat of severe thunderstorms has passed.
Weatherzone, a weather tracking service, warned "large areas of eastern Australia will be stuck in a stormy weather pattern...that could see several hundred millimetres of rain this week".
People living in Queensland's southeast have been warned to be on alert for flooding, with up to a metre of rainfall expected in some areas.
At least a dozen drivers have been rescued after their cars became submerged in the Gympie and Sunshine Coast regions, while a train has been derailed at Taveston northeast of Noosa amid the deluge on Wednesday.
The Queensland Fire and Emergency Service has sent emergency alerts to residents in the Gympie, Noosa and Sunshine Coast local government areas warning of flash flooding.
"Residents are being advised that roads and bridges may be impacted," the alerts say.