Parts of the NSW central coast and up into the Hunter are in line to receive "quite intense rainfall"  with the potential to cause flash flooding, the Bureau of Meteorology says.

The bureau on Tuesday issued a severe weather warning for a region centred on Newcastle and taking in places such as Dungog, Singleton and Gosford.

A coastal trough cradled between two strong high pressure systems is deepening, and could bring falls of more than 200 millimetres in places, starting Wednesday morning, Jordan Notara, a bureau meteorologist, said.

"The environment is very well set up for a very high intensity rain event," Mr Notara said.

“Along the coast, [there’s] potential for those more intense rainfalls to include flash flooding”.