Here in South Australia, the government relies on the manufacturer.
This is from a govt website:
Increased towing capacity and gross combination mass not permitted
A vehicle’s towing capacity is determined by the manufacturer with consideration to all aspects of the vehicle including:
engine and transmission capability
engine cooling package
wheels and tyres
towbar rating and attachment
ball weight limits.
It is considered that component manufacturers do not have the resources to test vehicles to ensure that these factors are adequately addressed.
Consequently, applications for increased towing capacity or increased gross vehicle mass will not be considered.
2014 PHEV Aspire, Arctic Silver;
Hayman Reese removable tow bar;
1.5kW Solar, grid connected.