There were "snow" covered streets from Blackheath down to Hazelbrook on Saturday when heavy hail hit after a sunny Anzac Day morning.
The Bureau of Meteorology forecaster Zach Berry-Porter said the hail storm on Saturday afternoon - which produced small stones in intense showers - was caused by cold air in the upper level of the atmosphere.
NSW State Emergency Services local controller John Hughes said the week's storm activity had kept SES volunteers busy.
The concentrated storm cell late on Saturday swept through the Mountains flooding the streets with hail and water.
"We've received 110 calls for property damage since last Tuesday right across the Mountains so it has been a busy week for volunteers being out every day and this Anzac Day storm didn't help," Mr Hughes said.
The heavy downpour lasted for about 30 minutes in Blackheath, Lawson and Hazelbrook after 2pm, causing extensive flooding and roof damage. Other parts of the Mountains remained dry and unaffected by the storm.
The SES received 10 calls for help from residents who reported roof damage and flooding, Mr Hughes said. A tree was also struck by lightning in Wentworth Falls.
"We were surprised by the quick onset of this storm as we still had teams working on jobs generated from last week's storm," Mr Hughes added.
While Saturday's hail storm caused moderate damage in the Mountains, suburbs in western Sydney were impacted the most with more than 400 properties damaged in Mt Druitt. Blue Mountains SES deployed teams to Mt Druitt, as well as to Gosford and Port Stephens to assist with ongoing storm damage operations.
The SES is monitoring another low pressure system that may form off the coast by week's end which could bring back more stormy conditions. The SES can be contacted on 132 500.
