An Adelaide mechanic who left two trainees alone to run his service centre has been banned from employing apprentices indefinitely.

Scott Thornton, who previously owned Ultra Tune franchises in Aberfoyle Park and Seaford, was found to have left two apprentices unsupervised for a week while he went on holiday.

"This is some of the most extreme conduct I've seen from an employer during my two years as a commissioner in this state," South Australian Skills Commissioner Cameron Baker said.

Thornton defended his actions, saying the trainees were capable enough to work independently.

"It was my understanding at the time that senior apprentices could work unsupervised," he said.

"They were more than capable of doing basic servicing, they had been in the trade for three-plus years," he added.

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