And with predictions around Lismore clearly wrong - after the storm came back inland and caused the river to overtop the levy on Wednesday - NSW SES media officer David Rankin is urging residents to be prepared.

"If you were affected by flooding in the last event, it's likely you will be again so prepare early and have all the information you need," he said.

"We are hoping people will start talking about their decisions early and prepare for the possibility that we might see more flooding here in the Hunter."

An incident control centre is already up and running 24 hours a day at Metford, with the SES at the helm. It will manage flooding in the Hunter and Mid North Coast.

Liaison officers from NSW Rural Fire Service, NSW Police, NSW Ambulance, NSW Resilience and the Australian Defence Force have a seat at the table and will be part of the response.

At this stage Mr Rankin said the Hunter could receive as much as 200 millimetres from this weather system.

"It's a multi-agency focus here," he said.

"We've got some pretty substantial weather coming our way in the next couple of days and we are keeping a very close eye on this system.

"It will take another couple of days before we see very heavy rain in the Hunter, but if it does come, it could lead to flash flooding.

"[Right now] there is a flood watch and a severe weather warning for areas of the Hunter coast."

Mr Rankin urged residents to closely monitor the conditions as things can change quickly.

"The way this storm is predicted to track should see it move out to sea around the Mid North Coast and then swing back in on the South Coast - and go around Sydney and the Hunter. But it is turning into a very deep East Coast Low so it's very difficult to predict its track.

"[On Tuesday] the Bureau of Meteorology had predicted that no more rain would fall in Lismore and then overnight they found upwards of another couple of hundred millimetres, so the storm that was predicted to head south headed back inland and we've seen the levy overtop [on Wednesday] morning."

Mr Rankin reminded people not to drive, walk or swim in floodwater and put their lives - and the lives of SES volunteers - at risk. He said the last flood event proved that some were not heeding the warnings.

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