LOCALISED downpours of up to 120 millimetres could douse some areas as intense storms loom on Wednedsday afternoon, the NSW State Emergency Service has warned.
The soaking would be welcomed by firefighters, who are working on dozens of firegrounds across the state.
With rain due in the coming days, the Bureau of Meteorology warned intense isolated storms drifting from the north west around Murrurundi towards Merriwa and Scone on Wednesday afternoon could bring large hail and damaging winds.
Downpours of 53 millimetres in 60 minutes have already been recorded at Nundle in the New England region about 3.45pm, the bureau said.
Showers and thunderstorms are forecast to increase across central parts of the coast and northwest on Thursday.
Peak rainfall totals of 30mm to 50mm per hour and localised 24-hour totals of 70mm to 120mm were possible, the SES warned.
The focus areas for the rainfall are the Hunter, Sydney, Illawarra and parts of the northwest.
"There's a risk of fallen trees particularly after bushfires in the Northern region, so we are urging people to monitor conditions, be alert and stay away from overhanging branches," SES Assistant Commissioner Nicole Hogan said.
"Trees that have been damaged by fire may be unstable and more likely to fall when it is windy and wet.
The Bureau of Meteorology advises that severe thunderstorms are expected over the central and northern slopes and ranges and adjacent coast from this afternoon and evening, bringing damaging wind gusts, large hail and localised heavy rainfall.
Forecasters were tracking a hot and moderately unstable airmass north and east of a trough through the northern inland Wednesday afternoon, which they said was likely to lead to deteriorating weather conditions and intense but isolated storms.
Assistant Commissioner Hogan said volunteers were prepared to respond to an increase in calls for assistance but urged the community to prepare their homes and families.
"Take those simple steps now to stay safe, including securing loose items around your backyard, trimming trees and branches and parking your car away from trees and powerlines," Assistant Commissioner Hogan said.
Conditions are expected to ease late Friday before another bout of thunderstorms is predicted for many areas over the weekend, shifting to the north on Monday.
Holidaymakers and travellers have been urged to plan and be vigilant to the conditions, and to keep abreast of updates via the state's emergency alert platforms and traffic advice.
To stay up to date with the latest warnings and information, download the Hazards Near Me app or visit the NSW SES website.
If you require assistance from the NSW SES call 132 500. If it's a life-threatening emergency, call Triple Zero (000).