Glenos,
Thanks for reviewing the numbers. A sanity check is always helpful.
I reviewed the report that i got from TasWater and found:
1. Calcium hardness was 12.8, not 128 as i originally posted. This was only measured once in a year.
2. The Mg numbers are correct but again, these were only measured once in a year, so the plausible explanation is that they were measured at different times.
Either way the water is pretty free of most salts/ions so that should provide a good base to make addition calcs off.