Ok I may off found that answer in an old tread answered by HK1837

These are the rules for NSW as Sandmans are pre manufacturers providing towing data:

If the vehicle manufacturer has not specified the maximum towing mass, the maximum towing mass is:
One and a half times the unladen mass of the towing vehicle, provided that the trailer is fitted with brakes which are connected and in working order, or
The unladen mass of the towing vehicle if the trailer
does not require brakes.

So for say a HZ V8 Sandman van at 2000kg GVM, take off load of 500kg leaves 1500kg. Multiply by 1.5 = 2250kg max trailer weight (minus towball weight). Towball weight comes off the 500kg capacity of the van, so if you have 150kg of towball mass (uncoupled trailer weights 2250+150kg) then you can only have 350kg in the van. This 350kg inlcudes driver, passenger and any load in the back.