It was understood the man was struck as the truck exited the Malaga site early on Wednesday.
A Transport Workers Union official confirmed the incident.
It was believed the man was a member of another union but had attended the protest to support TWU members during their 24-hour strike.
He was assessed at the scene but allowed to return to his car.
A spokesperson for Cleanaway said the company supported the rights of employees to undertake protected industrial action as part of the enterprise bargaining process.
"However, this action must be conducted in a safe and respectful way," the spokesperson said in a statement to AAP.
"Cleanaway called the police to clear the site to allow its safe operation.
"The company is aware and disappointed to hear of an incident that occurred this morning at our Malaga site.
"Cleanaway notes that its employees at this site are represented by the TWU but understands there was other union activity at the site today."
The union action, involving Cleanaway's Malaga, Guildford, Mandurah and Bunbury depots was launched over what the TWU said were attempts to force workers to do longer shifts and work weekends while cutting back entitlements.
"We have tried to reach agreement with Cleanaway, but they are determined to take away the overtime and weekends of their workforce," union state secretary Tim Dawson said.
"We held off on industrial action and tried to reach a settlement but it's clear that there is a national agenda from Cleanaway to blow up the hard-fought and won conditions in the industry."
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