A catastrophe has been declared in the wake of a fierce hail storm that pummelled the nation’s capital meaning thousands of insurance claims will be processed faster.

The Commonwealth Scientific and Industrial Research Organisation has been surveying the damage with several valuable research projects in tatters.

It is estimated at least 65 CSIRO glasshouses were damaged in Monday's storms, devastating crops of wheat, barley, legumes and cotton.

The research body had been growing the material on-site in an effort to improve crop sustainability, but those efforts now appear to have been in vain.

"For projects that potentially might have been close to the end of say two or three years work, that is really distressing," CSIRO chief operating officer Judi Zielke told the ABC.



"Unfortunately, most of those projects will be totally lost."

CSIRO Black Mountain. But looks like they may have missed the worst of the storm or this photo was taken early during 'the event' 🤷‍♀️ pic.twitter.com/HufqwVbF80 — Polly Marley (@marley_polly) January 20, 2020

Monday's weather event saw a record number of calls made to the ACT Emergency Services Agency - about 1,900 pleas for assistance were registered between midday and 8pm - more than triple the annual average.

Hail the size of golf balls left their mark on buildings and hundreds of cars in a 30-minute frenzy before the sun reappeared.

AAP

Emergency services worked through the night to respond to roof and window damage, fallen trees and electrical hazards.

Car windscreens were obliterated at the Australian National University and Old Parliament House, while a wind gust of 117km/h was recorded at Canberra Airport.

READ MORE Canberra pelted by hail the size of golf balls

At least two people were treated by ACT Ambulance Service for minor injuries from the storm.

The National Museum of Australia shut its doors after the storm tore external roofing, damaged shade cloths and caused leaks in corridors, the cafe and galleries.

Tom Swann/The Australia Institute

Animals were injured during the storm, with a koel, raven, galah, cockatoo and magpie all being treated at the Canberra Referral Hospital.

Flash flooding fears in NSW

Authorities are warning rainfall run-off in NSW's fire-affected areas may bring flash flooding filled with debris such as ash, soil, trees and rocks as temperatures are set to rise later in the week.

Severe thunderstorms hit Sydney, Newcastle and Wollongong on Monday.

AAP

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.

To get an idea of just how quickly heavy #rain can suddenly appear under current conditions, check out this from just before. From virtually nothing at Hornsby 1 min, to a blast of heavy rain across Sydney's nthn suburbs within 10 mins https://t.co/LmrJ3v2Eeo@NSWSES @nswpolice pic.twitter.com/ReUq39n0Tj — Bureau of Meteorology, New South Wales (@BOM_NSW) January 20, 2020

Joys of storm damage, fun fun waiting at Mortdale earlier to hopefully get to Wollongong @TrainLinkSouth pic.twitter.com/6w48mBfxLi — Nathan Harris (@Nathan19871) January 20, 2020

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.

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.

Crews have restored power to 10,000 customers overnight following the severe storms which hit Sydney yesterday.



We are still working to complete repairs so we can safely restore power to final 3,700 customers in Sutherland Shire.



📸 by Sutherland SES pic.twitter.com/uzLjSIR2qu — Ausgrid (@Ausgrid) January 20, 2020

At about 10.30pm on Monday 13,000 remained without power with repairs expected to continue overnight in the Sutherland 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.

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.

For crying out Cloud! #Kiama turned grey at 3pm today.



Don’t put your umbrellas away just yet – more thunderstorms are on their way and expected to cause havoc during peak hour for commuters on their drive homes tonight. Whats your plan for the commute home?#hailstorm pic.twitter.com/1RAA47J2Qs — NSW SES (@NSWSES) January 20, 2020

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.

READ MORE Large dust storms smother Central West NSW

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.

Victoria storm brings short fire reprieve

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.

A low pressure trough developing over central #Victoria and #NSW will trigger thunderstorms and widespread rainfall for the next 3 days. Heaviest falls expected in central and eastern Victoria.

Below is the forecast accumulated totals for the 3 days. https://t.co/A4eC8eXHQb pic.twitter.com/PtjR4LcHpD — Bureau of Meteorology, Victoria (@BOM_Vic) January 19, 2020

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.

AAP

The State Emergency Service 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.

EPA

A rise 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.

READ MORE Storms and floods pose a threat to bushfire-ravaged towns in Victoria

Additional reporting with AAP.