I’m guessing the tow car to trailer load weight is illegal there. Tow vehicle unladen weight should be more than the trailer and cargo weight!
It doesn't have to be, the only regulation is the trailer/tow vehicle combination must not exceed the tow vehicles plated gross train weight.
Including the driver, fuel etc - some assume that 1800kg say is just the trailer weight, but as @Bigherb mentioned - TRAIN weight
Known frequently in sunny Brizzal as a reversing sensor Ozziedog,,,,,,,,,, don’t get serious about much round here