Residents in fire-affected areas of NSW are being warned to brace themselves for flash flooding caused by run-off of ash, soil, trees and rocks.

Residents waking up on Tuesday morning will be reaching for mops after extreme weather patterns collided in the bushfire-fatigued state, after thousands spent the evening in the dark amid continuing power outages.

Drivers became trapped in cars as trees fell during the wild weather, and two people were struck by lightning.

It comes a day after regional areas of NSW experienced “apocalyptic” dust storms.

In the Australian Capital Territory, meanwhile, the phones of insurance companies have been ringing hot after a cold blast brought with it huge hail stones. 

Workers leaving offices – including Parliament House – were shocked to find the pelting hail had smashed car windows.

Some were looking on the bright side, pointing out that it was better than being smothered in the choking smoke that brought air quality levels to the worst in the world.

I've seen fire and I've seen rain, but I've never seen a freak hailstorm like the one that hit Canberra today. Hailstones the size of a fist. This is what it did to my car. I think Gaia is angry. pic.twitter.com/ST5kT1isnK

— Clive Hamilton (@CliveCHamilton) January 20, 2020

Thousands of cars damaged by Canberra hail storm. Back windscreens not built like the front. pic.twitter.com/SswIGNJblO

— Matt Roberts (@ACTcameramatt) January 20, 2020

Bureau of Meteorology forecaster Gabrielle Woodhouse said fire-affected areas could experience quick run-off and flash flooding. Roads could be 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.

Her warning comes as a 65-year-old man 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 on Monday.

Other victims of the storm included a 16-year-old boy who 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.

left: Parliament House, Canberra, January 5

right: :Parliament House, Canberra, January 20

(pics: AAP) pic.twitter.com/i7LWfrkHOv

— Josh Butler (@JoshButler) January 20, 2020

Work is underway to restore power to some 14,000 Ausgrid customers after strong winds, lightning and hail struck the Sutherland Shire and northern beaches areas.

As of 10.30pm Monday, 13,000 remained without power with repairs expected to continue overnight in the Sutherland Shire. Power outages continued on Tuesday morning.

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.

Golf ball-sized hail is shown at Parliament House on Monday in Canberra. Photo: Getty

In Victoria, the impact of flooding and debris running into waterways has challenged the battle against the flames.

“There’s significant chance for run-off today, off the ground, and for those streams and creeks to run quite hard with debris, rocks, sticks and the like,” SES deputy chief officer Alistair Drayton said.

These weather challenges were strongly felt in Melbourne’s eastern suburbs on Monday afternoon, where thunderstorms closed roads and delayed public transport.

The State Emergency Service has received 1824 calls for help since storms started hitting Victoria on Sunday.

About 1700 of them were in the Melbourne metropolitan area, mostly for building damage.

Spectators are seen as rain falls during the first round matches during day one of the Australian Open tennis tournament. Photo: Getty

Hot and windy weather is expected to return on Wednesday, pushing fire danger into the severe and extreme ranges again in some parts of the state.

A raise in the mercury levels will come with strong winds that could cause the blazes to flare up.

“The strong winds are the main driver, but we do see the temperatures climbing, especially on the north of the state, where temperatures are pushing up to the high 30s,” BOM senior meteorologist Richard Russell said.

Although light showers are expected to hold in East Gippsland until late Tuesday morning, the Bureau of Meteorology predicts the sky will be clear across the state by the evening.

The rain has brought some relief to the fire front, as the 14 active blazes in Victoria were all under advice levels that didn’t pose risks to lives and homes.

-with AAP