A world-first experiment in Sydney has found just how easily cars can be carried away by shallow water, making the crossing of floodwaters by drivers a potentially life-threatening decision.

Three men died after being swept away while trying to drive through floods in separate incidents in the ACT, the NSW southern highlands and Sydney's south-west during extreme weather earlier this month.

Engineers at the University of New South Wales used a test tank in Manly Vale on Sydney's northern beaches to replicate scenarios faced by stranded drivers.

They found that vehicles became vulnerable to moving floodwaters once the depth reached the floor of the car.

Principal engineer, Grantley Smith, said he was surprised at just how little water it took to make a car float away.

"People don't realise that even slow-moving water packs a powerful punch. Water is heavy: each cubic metre weighs about 1,000 kilogram," he said.

"If a house is exposed to floodwaters two metres deep and 20 metres wide - travelling at a steady 1 metre/second - the force is equivalent to being hit by a 40-tonne semi-trailer every 15 seconds.

"New cars in particular have really been made airtight because of the climate control systems that they have to keep the air conditioning temperature at the setting you'd like.

"Because they are airtight they are also water tight.

"So when you drive a car into water they just bob around like a cork. It's like having a boat on the water, they just float on the top of the water because no water can get into cabin of the vehicle."

Researchers found that even in low levels of water and slow flow speeds, floodwaters had a powerful enough force to make them float away.

Toyota Yaris moved by water 15 cm deep, researchers found

They found that a small car like a Toyota Yaris, weighing 1.05 tonnes, was moved by water only 15 cm deep and with a flow speed of 1 metre/second (or 3.6 km/h).

Mr Smith said that in 60cm of water the car completely floated away.

"So that Toyota Yaris becomes unstable in the floodwater, that is in moving floodwaters when there is about 20 centimetres of water around the car," he said.

Mr Smith said the researchers also tested a 2.5-tonne Nissan Patrol 4WD which they found would float once the water reached 95cm in depth.

"The 4WD — because it's a much larger vehicle, it weighs 2.5 tonnes — it starts becoming unstable when the water is about 45 centimetres deep, when the water is moving past at walking pace," he said.

"I was quite surprised about just how shallow the water depth were when the cars started to move quite easily in our tests.

"What we've found is that you really have to test it at full scale to get the buoyancy effects right to get the forces on the vehicle, measuring those correctly you really need to do that at full scale."

The tests are a world first, as previous experiments to understand the force of floodwaters have relied on using vehicle miniatures, rather than actual cars.