Rainfall run-off in NSW's fire-affected areas may bring flash flooding filled with debris as severe thunderstorms hit Sydney, Newcastle and Wollongong on Monday.
A 16-year-old boy was struck by lightning in the Blue Mountains on Monday afternoon, while a 24-year-old man leaning against a metal railing nearby 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 flown 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.
Hail, strong winds and lightning interrupted power to 2200 homes and businesses, Endeavour Energy said.
Properties in western Sydney, Macarthur, the southern highlands and the Illawarra remained without power after 7.30pm as emergency crews planned to work into the evening to make safe and repair damage from the storm.
Some trains on the T4 Eastern Suburbs and Illawarra line and the South Coast line were on Monday replaced with buses due to damage to a train caused by severe weather conditions at Sutherland, along with an object on the track at Caringbah.
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 quick run-off, flash flooding and roadways covered by ash and 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.
Temperatures are forecast to increase slightly closer to Wednesday and Thursday, with a spike in heat expected particularly for Thursday and Friday and a possible elevation of fire danger.
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.
Australian Associated Press