Authorities are warning rainfall run-off in NSW's fire-affected areas may bring flash flooding filled with debris including ash, soil, trees and rocks as temperatures are set to rise into the 40s later in the week.
Severe thunderstorms hit Sydney, Newcastle and Wollongong on Monday, while 5cm-wide hail caused catastrophic damage across Canberra.
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 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 10.30pm, 13,000 remained without power as repairs were expected to continue overnight in the Shire.
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.
Debris-filled rainwater poses major threat
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.
Penrith in Sydney’s west is expected to reach 42 degrees on Thursday while the CBD will hit 37. Bondi will reach 35.
Inland at Dubbo – where a giant dust storm swept over the town on Sunday – and nearby Narromine, temperatures of 40 are expected.
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.
Little hope of rain relief for Victorian bushfires
The summer thunderstorm that has brought heavy rainfall and hail over central Victoria is forecast to follow its path towards the east of the state, easing its conditions as it reaches bushfire-affected areas.
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.
But the impact of flooding and debris running into waterways has also 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 on Monday.
These weather challenges were strongly felt in Melbourne's eastern suburbs on Monday afternoon, where thunderstorms closed roads and delayed public transport.
The SES 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.
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.
Queensland cops soaking, more rain to come
Storms swept across Queensland’s southeast on Monday, with the worst of the weather between Brisbane and the Gold Coast.
At Thornlands, on Brisbane's bayside, a lightning strike was blamed for starting a fire that left a home severely damaged.
The storms dumped 67mm on Rochedale, in Brisbane's south, with Wacol, further west, recording 50mm. At the height of the wild weather, 20,000 properties were without power, but that was down to about 550 on Tuesday.
While not as intense as Monday, further storms are expected on Tuesday for Brisbane and the southeast.
Meanwhile drought-stricken inland communities are waiting for more rain after good falls in recent days filled water tanks and saw a statewide fire ban for national parks and forests lifted.
But the rain is no game-changer for Stanthorpe, on the Southern Downs, which has run out of water and is relying on supplies being trucked in.
There was direct rainfall over Stanthorpe's mains water reservoir, Storm King Dam, but a lack of runoff from catchments means the dam is still basically empty.
Mayor Tracy Dobie says six months of good falls are needed to break the drought.
Statewide dam levels rose by about one per cent after the weekend rain.
Little Nerang Dam in the Gold Coast Hinterland is near capacity, with Seqwater recording the dam at 96 per cent full, up from 72.5 per cent on Friday.
The Queensland Environment Department lifted the statewide fire ban on Monday.
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