expired Nulon Red Long Life Coolant Concentrate 5L for $26.99 (50% off) @ Repco
This is a Type A coolant used by most modern cars. This is a concentrate, so makes 10 litres. You can get 5l of demineralised water from woolies for $3.50 to use a a mixer, or use tap water if your tap water isn't too hard. But the extra $3.50 is probably worth it.