Hail as big as golf balls and ferocious winds of up to 117 kilometres per hour have torn through Canberra in a storm that had residents running for cover.

The storm, which swept across the ACT and New South Wales — including the firegrounds — smashed windows, dented cars and tore branches off trees, causing localised flooding.

The hail damaged countless windscreens across Canberra. ( Supplied: ESA )

More than 1,000 homes lost power, with outages reported in Florey, Griffith, Deakin and Kambah, and dozens more suburbs across the territory.

Since midday, the ESA had received a record number of requests for assistance, more than 1,900, with ACT Ambulance Service attending to two people for minor injuries sustained during the storm.

That was more than double what the service had received in the preceding two years.

ESA Commissioner Georgeina Whelan said they had withdrawn personnel from the firegrounds just over the ACT border where bushfires have threatened the territory in recent weeks.

"Obviously we have been stood up for weeks now in response to the threat of bushfires to the west of the ACT," Ms Whelan said.

"And based on the information we had received from the Bureau of Meteorology, both last night and this morning, we had withdrawn a number of our assets off the fireground and then we redirected a number of those assets along with our SES capability that we had pre-emptively stood up.

"We were responding to a number of the requests for assistance within a matter of minutes."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 m Summer hailstorm smashes cars and buildings in Canberra

Ms Whelan said the damage to the ACT had been "quite considerable", with the areas most impacted including Belconnen, the city, the Australian National University and the inner south.

"The damage ranges from electrical threats and power lines that obviously have fallen over, right through to a lot of damage to sky lights in homes, pergolas being significantly damaged and of course flooding," Ms Whelan said.

A specialist intelligence gathering helicopter was flying over Canberra in the storm's aftermath conducting a damage assessment from the sky.

"A combination of aerial surveillance and our damage teams will be out there on the ground assessing and prioritising the tasks — it will take us some time to get to all of those particularly challenging jobs," Ms Whelan said.

The ESA are responding to calls from across the ACT, as this map shows. ( Supplied: ESA )

The Bureau of Meteorology had issued a warning for "damaging, locally destructive winds, large, possible giant hailstones and heavy rainfall" due to hit the ACT at about 12:30pm, and advised residents to move cars undercover and stay indoors.

The BOM later reported wind gusts of 116 kilometres per hour.

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Hilary Wardhaugh, who was at the National Library of Australia when the hail started to fall, described it as an "unbelievable" sight.

"My friend and I were going to go outside and have our lunch and then we came out and saw it was raining and then the hail started," she said.

"It was like Armageddon, basically. Unbelievable. There were people running into the library but I'm really hoping that there's no one caught out in it."

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She said the hail badly damaged a number of vehicles parked at the library, as well as trees.

"It really only lasted 10 minutes and the amount of damage that it caused was truly unbelievable," she said.

"In this car park every single car is smashed.

"There are bits of plastic and glass everywhere and it's obviously hit the trees as well because there are branches and leaves everywhere.

"Some people are quite distraught, obviously, that their cars are so damaged but I haven't seen anyone who has been hurt which is good, so I think everyone is just trying to comprehend."

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Buildings were also damaged at the ANU, which announced it would close altogether on Tuesday as it assessed the impact on its facilities.

"Multiple buildings and services have been damaged by hail and we are aware that many cars are also damaged," an ANU spokeswoman said.

The office of the Australian Federal Police Association was flooded during the Canberra hail storm. ( Supplied: Australian Federal Police Association )

Videos on social media also showed the hail landing on Fiona Hall fern garden, an exhibit installed in the 1990s at the National Gallery of Australia.

The hail appeared to have damaged the some of the ferns, which are about 200 years old, according to the gallery's website.

The Royal Botanic Gardens also announced it would be closed on Monday and Tuesday due to storm damage.

And the Australian Federal Police Association (AFPA) shared photos that showed flooding and water damage inside Winchester Police Centre in Belconnen.