Updated 8.07pm: An emergency alert's been sent to residents in Newcastle and the surrounding areas in New South Wales - warning them to prepare for flash flooding.

Steven Pearce from the State Emergency Service says the alert has been sent to all mobile phones and landlines in Newcastle.

He says people need to account for young children, and elderly neighbours or friends.

"Under no circumstances should you attempt to drive or walk or ride through floodwater. It will put your lives and the lives of our responders too."

Heavy rain and cyclonic winds continue to batter the Australian state of New South Wales. 

Raging flash floods in the Hunter region have claimed the lives of three people.

The two men and a woman were reportedly trapped in their homes as flood water surged through the town early this morning.

The bodies were found in the Central Hunter region with police investigating the circumstances.

Premier Mike Baird says Dungog has been hit particularly hard.

"Obviously it's too early to say what has happened, but we do know that we have lost three residents. It's a tragedy for their family and friends and all of us are thinking of them."

The bureau of meteorology says severe thunderstorms are hitting Maitland, Raymond Terrace, Cessnock and waters off Port Stephens.

People in the area are being warned the heavy rain may cause flash flooding.

Steven Pearce says conditions are catastrophic with more than 4500 requests for assistance received.

In the hour to 9.45am, Tocal in the Hunter, recorded 112mm of rain.

In the 24 hours to 9am, Dungog had received 312mm, its heaviest recorded fall in more than 100 years.

Crawford had recorded 259mm, Wallsend 186mm, and Wahroonga 172mm.

NSW stricken by storm

Authorities in New South Wales have called on bosses to let their workers get home earlier than usual to minimise the challenges on the public transport system and roads in the wild and wet weather.

There are more than 250 traffic lights that are out across the road network while Sydney ferry services are not working.

Transport Minister Andrew Constance says they want people to get home before dark.

"We're urging people to get home early, and asking employers to be flexible with employees so we can get people home safely. People are going to have to be patient."

Schools are closed and emergency services are being inundated with calls.

State Emergency Services Commissioner Adam Dent says they've received more than four and a half thousand calls for help across the state.

"It is at the moment a challenging period for us in terms of the safety of our members, so we will be prioritising them. The life saving jobs will come first and then moving through as we can."

Emergency Services Minister David Elliott says emergency call centres are averaging 200 calls an hour.

"That just goes to show the human tragedy we've seen over the last 24 hours, and the anxiety that we expect."

Mike Baird says the weather is expected to deteriorate further this evening in the Hunter, the Central Coast and the Sydney region.

"Winds are averaging close to 100km an hour, that could increase between now and midnight."