The Perth mother at the centre of viral video showing her young son riding on her vehicle's roof rack has claimed the four-year-old boy is disabled and that she had no idea he was up there.

Several witnesses told 9News they saw the silver Holden Captiva travelling along Wright Road in Harrisdale, just before 6pm on Friday.

Security camera vision obtained by 9News captured part of the journey, showing the child wearing only a nappy, sitting upright on top of the car.

A Perth child has been seen, wearing just a nappy, sitting upright on the roof racks of a moving car. (Supplied)

Today the woman’s partner told 9News she had no idea their four-year-old son had climbed onto the roof rack for a wild ride unfolded across Piara Waters, the Southern River and on the nearby Tonkin Highway where the speed limit is 100km/h.

Eventually the Camillo mother responded to the flashing lights and beeping from other drivers and pulled over.

Gerry Georgatos, who provides help to traumatised indigenous people, is now representing the 36-year-old mother. He told 9News his client will front a media conference tomorrow to set the record straight.

The child's mother is understood to have been unaware that the child was on top of the vehicle. (Supplied)

“They're a salt of the earth family, solid, stalwart, and it was just totally unforeseen what happened,” Mr Georgatos said.

“What happened was inadvertent. It was without a knowledge. They're shell shocked. They're grieving at what could have happened.”

Mr Georgatos added that the family are “counting their blessings” following the incident.

“They’re praying to the heavens that nothing happened that would have injured their child, much worse.”

However, on social media dozens claimed that the mother has allowed the child to climb onto the roof rack before.

Police are now investigating the incident which left witnesses stunned. (Supplied)

“Bystanders have waved the mother down previously… her comment is that ‘he likes it’,” one said.

Police investigating the incident said after motorists alerted the driver, she "stopped and placed the child in the car before driving away".

"Canning Vale Police responded to the reports and located the vehicle driving into a petrol station on Nicholson Road, Harrisdale," a police spokesman said.

"Police observed three children in the car, two secured in child restraint seats, however it will be alleged that a four-year-old boy was unrestrained."

The mum has been summonsed to appear in court at a later date, and is facing a charge of dangerous driving, which only attracts a $3000 fine.

“There were a number of options available to police one possibly could have been reckless driving which carries a term of imprisonment of up to nine months,” lawyer John Hammond explained.