I'm not sure if this is right or not (if anyone knows better, let me know)...
When you view a multanova photo, you will notice when they zoom right out there are 2 dots at the top of the photo. If your numberplate is between the dots, you are the one who was being targeted by the radar and will receive the fine. There can be any number of cars in the photo, but only your plates will be between the dots and therefore you are the one who gets fined.
If they can't fine you if there are multiple cars on the picture, why do they even bother to set them up on multi-lane freeways, highways and the like?
Also, to the people posting up how they were booked under suspension and are getting a mate or family member to take the fine, use a bit of common sense before posting up silly things like that.
You are now lying to the police which is more of an offence than a speeding fine...