Flash flooding filled with ash, trees and rocks tipped to cause further devastation in NSW

Authorities are warning rainfall run-off in fire-affected areas of New South Wale may bring flash flooding filled with ash, soil, trees and rocks.

Severe thunderstorms hit Sydney, Newcastle and Wollongong on Monday.

A 16-year-old boy was struck by lightning in the Blue Mountains during the storm, and a 24-year-old man leaning against a metal railing nearby was also treated after lightning and hail hit the region.

Both were taken to Nepean Hospital in a stable condition.

Car windows were shattered in the storm and the ferocity of the downpour ripped branches from trees throughout the state

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.

Some 14,000 Ausgrid customers lost power after strong winds, lightning and hail struck the Sutherland Shire and northern beaches areas, the company posted on Twitter on Monday evening.

About 13,000 remained without power at 10.30pm, with repairs expected to continue overnight in the Sutherland Shire.

Golf-ball sized hail fell from the sky, shattering car windows and wrecking havoc across the state

A woman was spotted in Sydney CBD as the rain fell. A thunderstorm warning has been issued

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.

Emergency Services Minister David Elliott earlier on Monday warned of debris in rainfall run-off in fire-affected areas.

Car windscreens were shattered by the force of the hail which beared down on Canberra on Monday afternoon

The weather bureau confirmed the state was about to be hit by downpours (aftermath in Melbourne pictured on Sunday) 

A man was reportedly struck by lightning in the Blue Mountains on Monday afternoon. He was treated at the scene by paramedics before being rushed to hospital

‘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.

Wild weather in Canberra resulted in two people being struck by lightning and damage done to countless cars after a severe thunderstorm brought large hail to Canberra (pictured)

Golf ball size hail fell on the gardens at Parliament House (pictured) in Canberra on Monday after parts of regional NSW were battered on Sunday

‘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.

Mountains of ice piled up against doorways and footpaths in Canberra after the storm tore through

Car windows were shattered, people were caught racing for cover as massive hailstones (pictured) fell on Canberra on Monday wreaking havoc on the country’s capital city