Yeah definitely start a "paper trail" about this issue.
If someone has got the same vehicle as you for some dodgy use and have seen your car with the rego number the police are going to have a difficult time chasing it down, especially with the new rego plate recognition technology as the plates will match the description of the vehicle they're one.
I'd ask for the physical evidence of that vehicle on the tollway - then look for physical differences between that vehicle and yours. Roof racks, spoilers, bumper stickers etc.
Or change your plates. In vic you can buy replacement plates for ~$50. Painful but necessary in a situation like that. Also make sure you report plate theft/plate ID theft to the police as well, otherwise they'll still come knocking on your door.
All the best with it. Not a great start to the new year
EDIT: Also ensure a mate didn't borrow your car!
Cheers
Bennie