A motorist has shared her disbelief after witnessing what appeared to be a preschooler driving on Auckland's Southern Motorway in peak hour traffic today.

The woman, who asked not to be named, said she saw an adult female - head down and on a phone - with the child sat on her lap.

Photos supplied to the Herald appear to show the child with both hands on the steering wheel.

"I was following behind her and saw this kid jumping around on the front seat," the motorist said.

"He's jumping around and she's play-fighting and next thing he disappeared and I saw him jump into the driver's seat.

"She moved into the left lane and I got alongside her and I saw her texting on the phone and the kid driving the car, she wasn't even holding the steering wheel."

The motorist said the incident occurred in the Manukau area about 5pm.

Peak hour traffic was moving slowly intially, but "it cleared up and all of a sudden everyone was starting to do 60, 70, 80 [km/h]," the woman said.

Although driving herself, she said she felt compelled to take photos to serve as a safety warning to other motorists and perhaps to protect the child.

"I just couldn't believe it ... so yes I grabbed my phone a took a photo but I couldn't believe what I was seeing," she said.

"I didn't know what to do, obviously, it's not the first time she's done it because the kid knew what he was doing."

The woman said the child appeared to be "having a laugh" while driving.

A mother of six herself, the woman said she would never have thought about doing that with her children.

"I can't believe people would do that, it just frustrates me. I'm gutted that people would put their children at risk like that."