If you check your Owner's Handbook Section 8-2 (Specifications) it states:
"Coolant Specification 50% clean fresh water 50% extended life anti freeze coolant, conforming to specification number GM 6277M."
Holden recommend changing coolant every 60 months (5 years) regardless of km travelled.
I currently use NULON 'Red' Concentrated Long Life Coolant which (according to the label) lasts 5 years or 250,000 km. It DOES conform to GM6277M standards, it is Australian made and contains NO Silicate, Borate, Phosphate, Nitrite or Amine.
With the Alloytec you MUST use a coolant that conforms to GM standards otherwise you run the very real risk of major damage to Alloy components of your engine.
I always use Distilled Water (not very expensive for 5 litres) if/when replacing coolant. As per instructions 50% water and 50% coolant = 10 litres the cooling system capacity.
If you are not familiar with the procedure then I suggest you go to a workshop (I prefer to go to a radiator/engine cooling specialist as they can do a complete check of your system at the same time) where they have the appropriate pump system that will completely flush your engine. They then use required amounts of water and coolant and they are pumped into the system. Eliminates air pockets (can cause 'hot-spots' and overtheating). There's no reason why you couldn't buy your own bottles of water and coolant and provide them to the 'garage' - it would propbably help to keep costs down. Also pays to have your system pressure tested at the same time.