towing capacity

It all depends on the Gross Combination Mass of the vehicle. This is the total combined weight of Vehicle and Trailer that the unit is allowed to weigh.

BT50 GCM

My understanding is you can tow 3000kg plus the weight on the tow pin as long as it doesn't overload either the Gross Combination Mass (5500kg for Bt) or the Gross Vehicle Mass (3039kg for twin cab manual).
As an example a twin cab Bt (1900kg empty) with 700kg of people, fuel, assesories & luggage could tow a 5th wheeler weighing just 2.9 tonne as long as the a pin weight didn't exceed 400kg.
Lessen the load in the car & you can increase that of the 5th wheeler. At best if the car had two people & a tank of fuel you could tow 3.3 tonne as long as the pin weight was between 300 to 700kg.
This LINK may help.
Cheers Craig.................

wrapf
Was talking to a bloke over Easter about 5th wheelers and he was saying that if you put the hitch over or in front of the axle that becomes an articulated vehicle and appropriate licence required, but if mounted behind the axle just a car licence is required. Don't know if correct but maybe something to check out

Murray

Murray, that bloke is wrong. I have a heavy combination licence I need that to drive a semi trailer , a tag alony or a truck and dog. Tha last two are attached to the rear of the rear axle. In my case a heavy combination is required in Vic if the ATM of the trailer exceeds 4.5 tonnes.