The Brit was photographed cycling through Adelaide earlier this month (Jan13) while working Down Under on his new movie The Rover.
The picture showed the actor wearing a baseball cap instead of a safety helmet, which cyclists are required to wear by law in the state.
Locals authorities were alerted to the snap and contacted the Twilight star's management team to issue Pattinson with a warning.
A spokesperson for the South Australian Police says, "He was spoke (sic) to and told of the requirement to wear a helmet when riding a bike in the state of South Australia. He was just told of this requirement... and cautioned.
"Someone took a picture of him when he was out cycling so that is how we knew. If he does it again, he will be fined."
The fine is $90 (£59) plus a $60 (£40) victims of crime levy.