From the reports I've seen, Melbourne water is at near RO quality already. But to answer the original question, RO water quality is not typically at 0 TDS. There are low concentrations of any ions that are present in the feed water. In practice, RO removes a high percentage of the ions. The larger the ion, the better it is removed from the product water. Generally, divalent ions have larger diameter than monovalent ions and are rejected at a higher rate from the product water. So monovalent ions tend to make it through the membrane at a higher rate than divalent ions. But even with that, the transmission rates for all ions is quite low, between 1 and 4 percent. So a quick estimate is that 3 percent of any of the ions in the feed water make it through to the product water. (ie: 100 ppm of an ion in the feed water and 3 ppm of the ion in the product water). In the case of Melbourne's already very low concentrations, those concentrations in the product water are likely near zero.
I see that the Melbourne water quality can have a significant variation, so the real advantage of using a RO system is that it largely erases that variation. But given the reported average or typical water quality, RO is not needed or desirable for brewing purposes since we brewers DO want an appropriate level of ionic content in the brewing liquor to enhance the brewing process and beer flavor.
Building up the water to appropriate ionic levels should be fairly easy with this water. Assuming zero for all the ions would apparently only produce an error of around 10 to 20 ppm in many cases. But using more accurate values for the tap water or just assuming zero with RO water is better. Either of those water programs is fine for calculating mineral additions.
I recommend caution in reviewing the water treatment comments that were suggested by Tex above. The first thing to recognize is that chalk is not suitable for brewing use since it is not sufficiently soluble in water or wort to supply its alkalinity or calcium. At best, chalk has been shown to increase mash pH by 0.1 to 0.2 units and then it produces NO additional pH increase or alkalinity contribution...no matter how much more chalk you add. It just sits at the bottom of the tun. Far better alternatives for adding alkalinity to mashing water are slaked lime or baking soda. Both are very effective alkalinity contributors.
The baking soda brings to light another caution with the water treatment comments Tex pointed to. Sodium is definitely a troublesome ion when present in brewing water at elevated concentration. But at low to moderate concentration, it is quite helpful to beer flavor. This makes baking soda quite useful for adding alkalinity to the mash for those darker or more acidic grists. Adding baking soda at a rate that adds 40 ppm sodium, also adds almost 90 ppm alkalinity (as CaCO3). That is an acceptable tradeoff if the starting water has very low sodium concentration.
Another very questionable recommendation in the water treatment comments have to do with the chloride levels recommended. Chloride levels over 100 ppm may create minerally perceptions in the finished beer if sodium or sulfate levels are also elevated. I suppose that the chloride levels recommended in those comments are OK if the sodium and sulfate are kept low, but just be aware.
If you are interested, you can get a more comprehensive view of brewing water chemistry guidance on the Water Knowledge page of the Bru'n Water website.
PS: I thoroughly disagree with Tex's recommendation to use a temperature compensated pH meter in brewing use. Mash pH should ALWAYS be measured at room temperature since high temperatures will quickly destroy the pH probe. Therefore, the effect of temperature is not an issue when the measurements are properly performed. You can read more about pH meters and their proper use on the Water Knowledge page.
