Not law but when I worked in emergency services, it was something some people say you should have if you have a bad accident. It lets ambos and police etc know there is potentially a child in the car, if somehow they miss seeing the seat from the extent of the car being damaged. Horrible to think of but having said that we had a teddy one that kept falling off so it's now hanging on James' door handle!
This is also my understanding. I use to be in the SES and also work in the Emergency Service.