As long as your plate number/s is registered on your TAG account there will be NO Penalties/Extra Charge on your account when you driving without Tag. It will appeared on your statement as 'Undetected Tag' and they charge you a normal fee...no additional fee.
I had 3 cars (3 plate numbers) registered into 1 tag account....sometime I forgot to move the Tag to the other car when using the Toll...but yeah...that's fine.