I think you may have mis interpreted that.
I does say "vehicle and trailer combination", meaning that IF you are towing with a trailer and OVER the GCM of 4.5 you have speed limitations. NOT any trailer.
So if driving a Ranger or BT at 3t, plus towing your speed boat @1.4t total weight of 4.4t you can cruise on the posted limit (max of 110km per hr IN NSW)
I expected this argument
Interpretation ... it makes lawyers rich and when we lose, it makes us poor.
But I don't think I've got it wrong.
When reading the NSW road rules you have to use the definitions it uses. The Rules say that you have to use the definitions in the Act. Link.
The Act defines GCM as
GCM (gross combination mass) of a motor vehicle means the greatest possible sum of the maximum loaded mass of the motor vehicle and of any vehicles that may lawfully be towed by it at one time—
(a) as specified by the motor vehicle’s manufacturer,
(There are other provisions that are not relevant here.)
Note that GCM is not what the combination weighs on the day, which is what you used in your critique of my interpretation. GCM is the maximum permitted.
So my interpretation and examples stand. The BT50/Ranger's GCM is 6000kg. It doesn't matter if your BT50/Ranger and trailer as a combination weigh only 4.4 tonnes on the day you are towing, you are still captured by the definition of GCM (which for my example of Ranger/BT50 is 6000kg) and therefore your max speed is 100kph or the posted limit, whichever is less.
And this is only in NSW, of course. Other states have different rules.
I accept that this and the corollary of the Corolla (or Mazda 3 or whatever car whose GCM is under 4500kg) and the loaded trailer is maybe not what the legislators intended, but it is what they have written.