A Perth mother who drove with her four-year-old son on the roof rack of her car for 18 minutes on busy suburban roads, at speeds of up to 100 kilometres per hour, has been fined and had her licence disqualified.

Key points: Erica Shadforth reached speeds of up to 100 kilometres per hour with her son on the roof

Erica Shadforth reached speeds of up to 100 kilometres per hour with her son on the roof Her lawyer said she had been "completely unaware" the child was up there

Her lawyer said she had been "completely unaware" the child was up there Magistrate Steven Malley called the incident "carelessness in the extreme"

Erica Shadforth pleaded guilty on Tuesday in the Armadale Magistrates Court to charges of driving dangerously and failing to ensure that a child was properly restrained.

The mother-of-six was charged in January after other motorists saw the boy on top of the car as she drove along roads including Ranford Road and Tonkin Highway, where the speed limits vary from 80 to 100 kilometres per hour.

The court was told Shadforth was driving to a friend's house in Piara Waters from her home in Camillo, where she had put the child in the driver's seat before going back inside.

When she came back out to the car, she presumed the child had jumped into the rear seats where two of her other children — a two-year-old girl and a two-month-old boy — were strapped into their child restraints.

Shadforth's lawyer said she was "completely unaware" the child was on the roof. ( ABC News: Courtney Bembridge )

The court heard she drove for about 13 kilometres over 18 minutes with the boy in the roof rack, and was not aware of other motorists trying to get her attention to tell her what has happening.

Some witnesses had described seeing the boy losing his grip and coming out of the roof rack, before Shadforth stopped at an intersection in Harrisdale where she put the boy in the car unrestrained.

Her lawyer, Andy Matthews, described the case as very unusual and said Shadforth had sole responsibility for looking after her six children by herself, because her husband was working in Broome, where his family also live.

Mr Matthews said his client had decided to relocate to Perth to give her six children a chance at a better life, but she was going through "significant stress" trying to manage them.

He said Shadforth was "completely unaware" the child was on the roof but she had now taken steps to ensure a similar situation did not happen again.

"The boy will be put in the rear of the wagon where he does not have access to exit points," he said.

Shadforth reached speeds of up to 100 kilometres per hour with her son on the roof. ( Supplied: Channel 7 )

'Carelessness in the extreme'

Magistrate Steven Malley accepted that the offences were out of character but described them as "just mind-boggling".

"Seeing something like a four-year-old sitting on top of a vehicle is probably something that will stick in your memory for a long time," he said.

"It's not suggested that it was an act of ill will or recklessness, but I suggest it is carelessness in the extreme.

"When you leave young children in the car, you come back and count heads … they are children who do unusual things.

"The fact to some extent that he was not injured is obviously a positive."

Magistrate Malley said he needed to impose a penalty that sent a message to Shadforth and others to show care when they were on the roads.

He fined her $1,200, and while dangerous driving does not carry a mandatory licence disqualification, he banned her from driving for 3 months because of the seriousness of the offences.

Shadforth made no comment as she left the courthouse.