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A clean-up is under way after severe thunderstorms hit Sydney, Newcastle and Wollongong, with the New South Wales State Emergency Service responding to more than 800 calls overnight.
Some 14,000 Ausgrid customers lost power after strong winds, lightning and hail struck Sydney's Sutherland Shire and northern beaches areas, the company posted on social media on Monday evening.
Around 4000 customers remained powerless on Tuesday morning.
"The areas of Sutherland, Miranda and Caringbah were really impacted. We have teams out today, doing significant work ... we've done nearly 70 per cent of the jobs overnight," SES assistant commissioner Paul Bailey told ABC TV.
"It's been an amazing season ... the weather has been extremely unusual."
A 16-year-old boy was struck by lightning in the Blue Mountains on Monday afternoon, while a nearby 24-year-old man was also treated.
Both were taken to Nepean Hospital in a stable condition.
A 65-year-old man was was treated for multiple injuries and airlifted to John Hunter Hospital in a stable condition after a large tree crashed through a glass door at a house in Harrington on the mid-north coast.
NSW Emergency Services Minister David Elliott earlier on Monday warned of debris in rainfall run-off in fire-affected areas.
"Run-off from rainfall in fire-affected areas may behave differently and be more rapid resulting in flash flooding which may also contain debris such as ash, soil, trees and rocks," Mr Elliott said in a statement.
Bureau of Meteorology forecaster Gabrielle Woodhouse also said fire-affected areas could experience run-off, flash flooding and roadways covered by debris.
"Due to the fire and drought conditions, quite a lot of the vegetation is weakened and this means that trees and trees' branches are going to be much more likely to come down due to wind gust or a bit of heavy hail," Ms Woodhouse said.
Downpours have provided relief for parts of drought-stricken NSW in recent days and helped firefighters slow the spread of bushfires and build containment lines ahead of increased fire danger mid-week.
A spike in heat is expected for Thursday and Friday.
The SES said NSW residents could help prevent storm damage by trimming back overhanging branches, cleaning gutters and downpipes and securing or putting away loose items around the yard or balcony.
Wind warnings are in place for the Byron, Coffs, Macquarie and Eden coasts.