Yes, you are right in saying that Adelaide water is average (not Quite as bad as your description, but not as good as Melbourne water). It has a habit of killing XTOL when you least expect it. But, alas, I mix my developer with bottled spring water (generally the cheapest I can find in the super market at the time). I also filter all other water that is used during the process.
In some countries, spring water is "mineral water" prized for the minerals that are useful for the human body. Here in Australia, the term "mineral water" sort of disappeared when the mineral content was considered unimportant to the customers. But if the water you're using actually comes from a spring it could have some dissolved substances in it which you might or might not want, perhaps calcium. Lots of bottled water here is labelled "purified water" or some such thing which might be demineralized water, I'm not sure. So it would be suitable for developer (more so than for humans perhaps). But I'm loath to use anything without knowing what's in it.
Here in Melbourne the water is fairly good (not as good as in the old days though) so I've not been impressed by my daughter buying bottled water which is nothing special. So I told her that the highest honours in the field of advertising are awarded to those who convince people to pay for what they used to get for nothing. I think it did have some influence on how she spent her money.