The State Emergency Service says with up to 150 millimetres of rain expected to fall in the Hunter over the next 24 hours, a flood watch has been issued for local rivers.
The Weather Bureau has advised that a low pressure system has developed off the north coast, bringing heavy rain over the next two days.
SES acting Hunter region controller Ken Speer says all rivers in the Hunter region could be affected.
"There's a current flood watch for minor to moderate flooding for the Hunter Valley," he said.
"We are expecting between 100 and 150 mils of rain, widespread across the Hunter today.
"Over the next 24 hours, that's going to occur.
"We had the rain a couple of weeks ago, so that has certainly filled up all the storage areas that were available throughout the Hunter Valley," he said.
"As a result of that, any rain now is going to be run-off into the rivers, and the potential is there for flooding to occur."
© ABC 2013
05:48 EST A second storm cell developing off New South Wales is expected to hit northern parts of Sydney and the Central Coast in the next few hours.