A Perth bus driver has been sacked and a police investigation is underway after a young boy suffered serious injuries from being trapped by a bus door and dragged along the road.

Jeremy Thompson was getting onto the vehicle when the bus door shut, trapping his foot, and drove off.

The 12-year-old suffered serious injuries, which reportedly included a fractured pelvis, back and hand.

The Public Transport Authority confirmed the incident happened near the Mandurah shopping centre on the afternoon of May 14.

"The bus driver has since been sacked by our contractor," a PTA spokesperson said in a statement.

"As the matter is under investigation by police, we cannot comment further at this time. The PTA and its contractor are assisting with the inquiries."

Boy, family traumatised

Megan Krakouer, acting as a spokesperson for the family, told the ABC Jeremy was traumatised by what happened.

"He wakes up at night thinking about what could've been," she said.

Ms Krakouer said the family contacted her, panicked and upset, shortly after the ordeal.

"They were concerned about the inaction of Transperth and the police," she said.

The WA Police said the investigation into the incident is ongoing and no arrests have been made so far.

WA Premier Roger Cook described the incident as "horrific".

"This is a very distressing report and I believe the bus driver involved has been stood down," he said.