The Bureau said a complex low pressure system is expected to deepen over the western Tasman Sea and intensify into Friday, bringing with it gale-force southerly winds from Seal Rocks to Ulladulla.

Thunderstorms and localised heavy rain are possible which could lead to flash flooding, especially in the Illawarra.

The Bureau warns damaging winds, averaging 55 to 65 km/h with gusts of 90 km/h, are forecast for the Hunter, Central Coast and Illawarra regions.

Dangerous surf conditions will also develop during Friday, with the Bureau warning rock fishing, swimming and surfing will be hazardous.

In the two days to 9pm on Thursday, May 21, 2015 the Bureau of Meteorology said rainfall totals across the Upper Hunter were solid.

Totals were generally consistent across the board, with Merriwa recording 42mm; Baerami 39mm; Murrurundi 38mm; Bylong 36mm; Cassilis 34mm; Parkville 35mm; Bunnan 25mm and Aberdeen 24mm.

Over the same period Liddell, just south of Muswellbrook, had 20mm; Denman 18mm; and Singleton 16mm.

The State Emergency Service has advised people in the severe weather warning areas to move vehicles undercover or away from trees; secure loose items around the home; and to keep clear of any fallen power lines.

The SES has stressed that people should not drive, ride or walk through floodwaters and keep clear of creeks and stormwater drains.