The lack of manufacturer written advice, limitations, cautions, or warnings (written or aural) about autopilot response to manual pilot control inputs meant that pilots may be unaware that their actions can lead to significant out of trim situations, and associated aircraft control issues.
Response to safety issue by Garmin
The avionics manufacturer indicated that since all autopilots work the same way, it was common knowledge for pilots not to manually manipulate the flight controls with the autopilot on. Therefore, any advice, limitations, or warnings about the issue would not be required. They also indicated that the presence of a limitation, warning, or caution is generally left up to the certifier of the equipment in the airplane [in this case Cessna].
ATSB comment/action in response
Advanced avionics systems and autopilots are now fitted to many aircraft used by relatively inexperienced pilots. Therefore it is important that every opportunity is taken to inform them of the potential danger posed by manual manipulation of the flight controls with the autopilot on.