Just go with the genuine Holden coolant you can get from auto one and other auto stores. It comes premixed. Might be an idea to flush your old system though if you are unsure what is currently in there. I believe even using the same colour coolant can get you in trouble.
Clacks
The genuine Holden coolant is a concentrate, and thats the one you need. Just pick it up from a Holden dealer, its about $39 for the 5 litre bottle, and about $12 or so for the 1 litre. Definately a good idea to flush the block and radiator completely before adding in the new coolant. Also, NEVER mix coolants, the chemical properties are different for each one, and can have a bad effect on the cooling system, sometimes even increasing corrosion.
For the V6 you will need 6 litres of the concentrate, and 6 litres of clean water. The capacity of the system in the VS V6 is 12 litres, which includes radiator, block, hoses etc and the reserve tank.
The best way I have found of getting all the concentrate in is to make sure your heater is set to full heat, remove the top radiator hose and thermostat, flush radiator, and remove bottom hose, then flush the block by putting the hose into the thermostat opening, and wait until clean water is coming from the bottom radiator hose. Then hook up the bottom radiator hose, pour the entire 5 litre concentrate into the radiator, then put the thermostat back in, bolt it all back together, put another 750mL concentrate into the reserve tank, then just fill the rest of the system with clean water, that way you have the correct 50/50 mix of coolant concentrate and water.