A tree has fallen on Northbourne Avenue, affecting traffic.

Video posted online also showed a roof leaking at Westfield Belconnen, and the hailstones were described as being "like bullets" as they punctured the roof of the National Film and Sound Archive.

There are multiple reports of smashed car windscreens, broken windows, dented vehicles, trees down and branches shredded.

The Bureau of Meteorology said it had had reports of 4-5cm hail, and warned further storms were possible on Monday afternoon.

The bureau issued a severe warning on Monday morning as heavy storms formed west of Canberra with a northwesterly forecast to push the system into the city.

Bureau forecaster Abrar Shabren said then that the system would be fast-moving and winds could bring the most damage. He warned of golf-ball-sized hail stones.

"Damaging winds are the most likely phenomena with fast-moving thunderstorms," he said.

"Short sharp bursts of rainfall are expected."

Mr Shabren said the extreme weather was forecast to hit the ACT from early to mid afternoon on Monday.

Emergency Services Agency ACT commissioner Georgeina Whelan said the ACT was in for an "intense couple of hours" and storm activity was expected to hit most parts of the territory.

"We normally see some storm activity to the north or the south of the ACT, not necessarily the entire region [but] today we could actually see from the north of Canberra right through to the southern tip of our borders actually being impacted by storms, winds and lightning strikes," she said.

Chief Minister Andrew Barr said the ACT State Emergency Services had been stood up to respond to the impending storm.

People have been advised to move their cars under cover or away from trees, secure loose items around the house, yard and balcony, keep clear of creeks and storm drains and not to walk, ride or drive through flood water.

The warnings come as damaging winds produced by thunderstorms across central NSW whipped up dust storms that turned daytime into night.

Bureau meteorologist Rose Barr said significant rain on Sunday was concentrated across central and northern parts of the state, and east of the ranges.

Many towns on the mid-north coast and the northern rivers region received between 100mm and 180mm from 9am to 10.30pm on Sunday.

In the southern part of the state, high winds meant storms raced overhead quickly, resulting in lower measured falls.

Downpours have provided relief for parts of drought-stricken NSW and helped firefighters slow the spread of bushfires, and build containment lines ahead of increased fire danger mid-week.

"[On Monday] we have a very active thunderstorm day forecast, particularly across southeastern and central-eastern parts of the state, as well as parts of the southwest slopes," Ms Barr said.

Winds will shift and come more from the north and west mid-week, bringing drier and warmer air on Wednesday and Thursday.

"That means on both of those days we may end up seeing fire dangers increasing again and causing more problems for our fire agencies," Ms Barr said.

But rain will return on Friday and the weekend.

Canberra's air quality improved on the weekend and on Monday morning was rated very good at the Monash and Florey air quality stations, and good at Civic.