The New South Wales Hunter and Central Coast regions have been battered by large hail stones this afternoon.
The Bureau of Meteorology has issued a severe weather warning, with storms detected on the radar near Swansea and Lake Macquarie just before midday.
Senior meteorologist Angus Hines said the severe hail was due to a cold front moving north through the coastal regions.
"Near that cold front we've seen some big thunderstorms develop just in the last hour or two," he said.
"Those thunderstorms are generating some really large hail just for those coastal parts of the Hunter district."
Storms move north-east
The weather bureau said storms were predicted to travel north throughout the afternoon, with places such as Sydney, Belmont, Swansea, and Newcastle also impacted.
There have been reports of golf-ball-sized hail in suburbs such as Charlestown.
Mr Hines said forecasters would track the thunderstorms throughout the afternoon.
"We've got a severe thunderstorm warning out covering the area likely to be impacted by these large hailstones over the next half an hour to an hour," he said.
"Our severe weather forecasters will be keeping that up to date."
Activity to ease
Mr Hines said the storms were fast to appear, but they should start to ease later in the afternoon.
"Through the rest of the afternoon we do expect these thunderstorms and all the activity with the hail to pull offshore out over the water and weaken away," he said.
"But it could still be another hour or two for some of those coastal places so do stay up to date and check if you're in the severe weather warning area."
State Emergency Service (SES) chief superintendent Andrew Cribb said crews were responding to hundreds of calls for assistance and more were expected to come through as storms travelled north.
"The storm seems to have moved through. We have taken more than 200 calls for assistance and that is rapidly increasing," he said.
"The teams have now deployed into the field and are prioritising those jobs as we try to rectify the damage caused to properties."
Mr Cribb said the damage was varied and crews had "seen everything".
"We've seen flash flooding with backed-up drains and roadways covered. Luckily to this point we haven't had any flood rescues," he said.
"The majority of jobs have been broken skylights, broken roof tiles, and damage from rain coming into properties through the roofs.
"We have had one carport that has received significant damage."
The SES advised that people in the severe weather warning region should move cars undercover, unplug computers and appliances, and avoid using phones during a storm.
People are also advised to not walk, ride a bike, or drive through floodwater.
