I live in Perth as well the reason we use antifreeze over here is it contains corrosion inhibitors, these stop the electrolysis happening between the two disimilar metals(aluminium head and cast iron block) if you use distilled water only the head will slowly eat away and cause build up in the cooling sytem. Most heads last about 5-7 years,depending on the quality/ density of the aluminium casting, if you don't run a corrosion inhibitor. Then you are up for either a new head or having yours welded and re-faced
