A Perth mother at the centre of a viral video showing her four-year-old son riding on her vehicle's roof racks has fronted the media and, through a representative, denied any knowledge of the boy's wild ride.

Speaking on behalf of the mother-of-six, Gerry Georgatos from the National Indigenous Critical Response Service said it's obvious the woman - who only wants to be known as Erica - had no clue her autistic son had climbed onto the roof of the family's Holden Captiva.

"The mum was blindsided, didn't see it. As soon as she would have been aware of it, she would have stopped the vehicle," Mr Georgatos said.

A Perth mother at the centre of a viral video showing her son riding on top of her vehicle today fronted media for the first time. (9NEWS)

"He's four years of age, very adept, he obviously unrestrained himself and scurried out."

The boy's 10-week-old baby brother and 19-month-old sister were also in the vehicle at the time.

But Mr Georgatos said it wasn't fair to ask whether 36-year-old Erica was doing any sort of checks on her children, such as looking in her rear-vision mirror during the journey.

The woman's four-year-old son was filmed wearing just a nappy while sitting on top of the moving car. (9NEWS)

In a statement released through the National Indigenous Critical Response Service, the mother-of-six instead suggested her son's 'adept' nature was to blame for him being on the roof. (9NEWS)

The mother also denied reports that surfaced on social media about the child being seen on previous occasions doing the same thing. (Supplied)

That is despite witnesses saying the trip spanned several suburbs and included Tonkin Highway, where the speed limit is 100km/hr.

"That's not a fair question because there has to be some presumption, has to be some cognition that the child is missing from the back of the vehicle," Mr Georgatos said.

He also denied social media reports from people who claim to have seen the boy riding on the vehicle's roof several times before.

The mother has maintained she was completely unaware her four-year-old son was on the roof of her moving car. (Supplied)

In the statement, the mother claimed 'we've got the makings of a trapeze artist' on their hands, and claimed if she had known he was on the roof, she would have immediately stopped. (Supplied)

"Not true whatsoever. The child hadn't done it before, had not been on a moving vehicle... (the comments) are disturbing and they're cruel and they're insensitive," he said.

Erica is now fighting a charge of dangerous driving, and the boy's father has removed the roof rack basket from the family's car.

"You know we've got the makings of a trapeze artist," Mr Georgatos said.