He said the man working was knocked to the ground and was "shaken up".
The mechanic said the cash register had been ripped out.
The incident was "nothing new to us," he said, with the last event like this happening six months ago.
Police are appealing for information from the public about the incident.
A police spokeswoman said a group of male offenders reportedly entered the service station around 6.30am and assaulted a staff member.
The alleged victim did not require medical attention.
She said the group allegedly stole cigarettes before leaving in a dark silver/graphite-coloured Subaru Forester.
The group is believed to have also used a silver Nissan Wingroad vehicle, she said.
The Subaru vehicle was found abandoned a short time after, near Turner Drive and Thomas Cres.
Police are making a number of inquiries to find the offenders, including reviewing CCTV.
Police are calling on anyone who was in the area at the time who may have seen the group of offenders or the vehicles to get in touch.
Anyone with any information can contact the police on 105, quoting file number 220217/9448. Information can also be provided anonymously via Crime Stoppers on 0800 555 111.