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 has issued a severe weather warning on Wednesday morning for a region centred on Newcastle and taking in places such as Port Stephens, Singleton and Gosford.

Dungog in the Hunter Valley had already received 68 millimetres in the hour from 4.10 am to 5.10 am, the bureau said, revising an earlier report of 117 millimetres in an hour.

A severe thunderstorm warning was issued by the bureau on Wednesday morning, with heavy rainfall that may lead to flash flooding possible "over the next several hours. Locations which may be affected include Forster, Seal Rocks, Bulahdelah, Nelson Bay and Raymond Terrace".

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.