Le Truck?

GVM and licensing
In Australia, a car driver licence is limited to driving vehicles up to a maximum GVM of 4495 kg (9900 lb). Beyond this, a different class of licence is required. A vehicle with a GVM under 4495 kg is termed a light vehicle, while those over 4495 kg are termed heavy vehicles.

Many models of truck, for example F-Trucks, Silverado or GMC Sierra Denali, are manufactured to have a GVM rating of 5000 to 7000 kg (11,000 to 15,400 lb) but sold with the option of a GVM of just under 4495 kg so that they can be driven on a car licence. This is called a GVM downgrade.

As a government-approved provider, Performax International and its dealers may be able to assist with GVM downgrades on some vehicles to allow driving on a car licence. This is covered under state legislation, therefore differs from state-to-state.

A F150 can be driven on a car licence. A F250/ F350 can also be driven on a car licence. A F550 No. U.S. tow Ratings are not acceptable
Anything slightly over 4995kg goes from a Ute to a light Truck. No big deal.Most RVers would need a light truck licence for a larger Motorhome, it applies to Pickups, trucks, motorhomes

You have probably been given the correct info re licence. Apparently if you are purchasing a new F350 you need to make sure it's de-rated to 4995kg GVM or you will have to get a light truck license to drive it and it will have to go through a machinery inspection every year like any other commercial truck. Probably no way around it with an F450