Except that usually you do not really drink water from your heaterStainless. Aluminum will probably exceed the US National Drinking Water Standards for aluminum in drinking water. I don’t know the standards in Australia.
I have aluminum fresh-water tanks. I collected an aliquat of water and sent the sample to a certified laboratory for metals analysis. The results for aluminum were quite high.
Very pleased with the IsoTemp SPA series of water heaters. Exterior is Poly, tank is Stainless.