A Perth mother has been fined after she was captured driving with her young son on the roof racks of her car which was travelling up to 100km/h.

Earlier this year, vision showed the four-year-old boy, wearing only a nappy, sitting upright on top of the car as it raced through the city.

Vision showed the four-year-old boy, wearing only a nappy, sitting upright on top of the car as it raced through the city. (9news)

The mother pleaded guilty to dangerous driving and failing to properly restrain a child. (9news)

The court heard the child climbed onto the roof while his mother, Erica Phoebe Shadforth, went inside.

When she returned to the car, Ms Shadforth was unaware he was there and they drove for almost 20 minutes where witnesses saw him lose his grip and almost tumble out of the basket.

Eventually, she pulled over and placed the boy unrestrained in the car with two other her other children.

The judge called the incident 'mind-boggling'. (9news)

The court heard the child climbed onto the roof while his mother went inside. (9news)

Ms Shadforth’s lawyer told the court while it didn’t excuse what happened, the mother was under extreme stress at the time, trying to cope with managing six children between nine and five-months-old.