Simply put, Gross Combination Mass (GCM) is the vehicle’s towing capacity. In other words, it measures the total weight that a truck can carry and tow including the weight of the mass the vehicle is towing, the weight of the vehicle itself, and any payload (the weight you can load into a vehicle including all passengers, luggage/cargo, and accessories). GCM is determined by the vehicle’s Gross Vehicle Mass (GVM) and Aggregate Trailer Mass (ATM), which is the maximum towing weight of the trailer or caravan.
Each state has its own rules governing GCM upgrades for all relevant vehicle types. Until June 2018, upgrading one’s vehicle to carry or tow more weight (increased GCM) was relatively simple – the easiest way to do so, for instance, is by upgrading suspension.
In June 2018, however, the Australian federal government banned any future weight upgrade approvals on unregistered vehicles and many state governments such as Victoria followed suit by adding conditions or restrictions to their GCM upgrade standards.