In all States and Territories of Australia, the darkest legal window tint permitted on a vehicle is one with a VLT (visible light transmission) level of 35%, on all vehicle windows (excluding the front windscreen, which is not allowed to have any window tint with the exception of the visor strip across the top). The northern Territory and Western Australia are the only exceptions. In the NT you are allowed a minimum VLT of 15% for windows behind the driver; and in WA you are allowed 20% VLT on windows behind the driver.
So what's problem? What people don't realise is that most vehicles already have a slight factory installed tint in the glass. This varies from model to model, and it really should be considered when adding after-market tint to a car, otherwise you run the risk of having illegally dark windows. But before we discuss these problems, lets first examine the maths and see how the problem can happen.
If the factory glass on your car already block 30% of light, when a film with the "darkest legal tint" of 35% is added to this window, it will emit only 35% of light into a window that is already only emitting 70% of light, so the end VLT will be finalised by the addition of both VLT ratings.
This needs to be taken into consideration because if a driver accidentally fails to comply with tinting regulations, the result can be a fine. But worse still, if a vehicle is involved in an accident and its illegally dark windows are considered by the court to be a contributing factor, this could mean the nulling of your insurance policy, leaving you exposed to the full financial implications of the accident. Additionally a criminal charge could apply if property is damaged or people are hurt.
The final thing to consider is that by modifying a vehicle with illegally dark windows, the vehicle is deemed to be un-roadworthy, which means the driver can't drive the car again until it has been put through roadworthy testing, in which case the illegal tint will have to be removed. That's why the combined VLT of both the glass and film really should be considered when you're selecting the appropriate tint for your car and that won't happen unless you use a very professional installer who will measure the VLT ratings of your glass before selecting after-market film.