This is not a new regulation. However, it applies only to 50mm balls. Other couplings can be at any height. A 50mm ball may be used at a different height provided its height can also be adjusted to the range specified.
Ball couplings used on trailers with an ATM of up to 3.5 tonnes must comply with Australian Standard 4177-3 and be installed so that when the coupling and towball are connected to the laden towing vehicle; the height to the centre of the tow ball above the ground shall measure between 350mm and 460mm (or be capable of being adjusted)."
"Ball couplings on towbars are required to be installed so that the height of the centre of the body of the ball coupling is between 350mm and 420mm from the ground when laden (Refer to ADR 62/01). Alternatively, if complying with the requirements of ADR 62/02 the maximum height of the centre of the body of the ball coupling may be increased to 460mm. However, the ball may be installed at any other height, provided it is also capable of being adjusted to at least one height within the 350-460mm range."