Canberra man Tom Corra said water was higher than the car's windows, when he arrived at the scene just after it was swept into the middle of the dual carriageway about 6.20pm.

The road was closed about 10 kilometres north of Canberra from 7pm in the northbound direction with traffic diverted at Sutton onto the Old Federal Highway until about 9.30pm when one lane on each side was reopened.

"The water was rising pretty quickly," he said.

"Someone said 'there's a possibility we might have to drag a car out, there's people stuck in there'.

"On the southbound lane you couldn't see the guard rail... the water was probably a metre high."

ACT Fire and Rescue superintendent Pat Jones said his crew were initially told up to four cars were caught in the water.

Luckily, one member of Gungahlin unit tasked to the job was trained in swift water rescue, which is not a requirement for ACT firefighters.

"They used a rope technique to quickly and safely access the car," he said. "We had a lot of people out there who were required to do a lot of work in a short period of time."

Canberra airport recorded 6.6 millimetres of rain as of 9.30pm after warnings of a severe thunderstorm in the afternoon.

But closer to the NSW border in the ACT's north, there was about 18 millimetres of rain and hail as large as two centimetres near Bungendore about 5.30pm.

Mr Corra said fortunately there was an off-duty police officer at the scene on the Federal Highway and the rural fire service soon arrived to winch the car out of the waters.

"They got them out pretty quickly, but there was also the issue of all the other cars trying to come south down the highway into Canberra, trying to get through the floodwaters," he said.

"They looked very shaken, they were probably 40 or 50-years-old.

"It was crazy, I was just out taking photos of the storm and next thing you get caught up in that."

He said he was at Sutton where it was only drizzling before driving about 800 metres down the road to be confronted with the highway covered in water.

A Goulburn police spokeswoman said there had been "multiple" minor vehicle crashes, but no reports of injuries as of 8pm and the only southbound lane opened at the time was covered with debris from the flooding.

Earlier motorists had been warned to drive with "extreme caution" on the Federal highway due to the severe storm and flooding between Sutton and Currawang, about 14 kilometres south-east of Collector.

Mr Corra said there were many cars getting stuck in the waters as they tried to drive through, but the water had begun to subside by the time he left about 7.45pm.

Two vehicles from Queanbeyan SES responded to help the Sutton SES with five flood rescues as of 8.30pm.

Sutton Road was also closed about one kilometre north of the highway but drivers were still attempting to get through the floodwaters, a NSW police spokesman said.

Emergency crews were also kept busy with several small bushfires started by lightning strikes around Canberra.

The ACT Rural Fire Service dealt with at least four fires in the Kowen Pine Forest, with the first reported at 5.24pm.

Over the border, the Queanbeyan Rural Fire Service were alerted to a fire at Carwoola at 5.30pm and a spokesman said lightning had caused about three other small blazes including a single tree fire near Bungendore and a small fire on Old Cooma Road at Googong by 7.30pm.

He was hopeful rain would continue to help emergency crews fighting the fires.

There was also reports of a fire near the Barton Highway at Murrumbateman.

Despite the rain and severe storm warnings earlier in the evening, an ACT Emergency Services Agency spokesman said there had been less than five calls for assistance as of 9.30pm, but there at least three power lines had gone down in the ACT.