The owner of Mower Magic said he was shocked when he arrived at his business on Rouse Road in Greenfields after 7pm on Sunday to find an intruder on the premises.
Pat Fisher (49) and his son (29) arrived at the business because the alarm was activated.
When they got there they found a white Isuzu truck with a trailer had reversed through the rear door of the workshop.
Mr Fisher said everything happened “so quick” from that moment on.
“I’m concerned about the other guy,” he said.
“He had a bit of a slip over as he went out the door.”
Mandurah detectives are investigating the incident.
They claim Mr Fisher’s son and the intruder became involved in a physical altercation that resulted in the intruder falling to the ground unconscious.
Mr Fisher said he called police and St John Ambulance immediately after. He said this was the first time his business had been targeted in this way, but there had been issues at other businesses in the area.
He said the intruder may have been after power tools.
Other businesses in Mandurah have had chainsaws stolen in recent weeks.
Detectives said inquiries were continuing.
They would like anyone with information to call Crime Stoppers on 1800 333 000 or make a report to www.crimestopperswa.com.au.