A note to a member of the public and reported to council last Tuesday read that "this vehicle has been photographed and identified as causing an offence".
It then went on to tell the recipient of the letter that to avoid further action, they should go in person to the City of Cockburn.
Community services manager Rob Avard said the fake letter was a one-off, but added that any incident like this should be reported to police.
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READ NOW"All information and advice notices distributed to any resident or vehicle owner from the City's services areas would contain the City's emblem or watermark, the officer's identification number or a name, including a contact telephone number," Mr Avard said.
"The City encourages residents to report any documents similar to those in the photograph to the City immediately."