Hi Andrew,
I am basing my assumpions for Bribie on the techniquies i currently employ to water thats has a marginal mineral concentration already. Have you seen the water reports for Mebournes water? Everything is 7ppm or less except for carbonates which sits around 15~. Thats about as devoid of minerals/close to RO water as you are going to get.
If you are brewing a pale ale with no acidic malts, there is no reason why you should be adding alkalinity in the form of (bi)carbonates. Its just not required and its possible you will stuff your mash if there is not enough hardness to buffer it.
As i had already noted which you may have missed, adding a few L back of your base tap water unadulterated (carbon filtered if you are so inclined), 2-5L should suffice to give you some trace under 5ppm, which is all that you would need. There is no requirement for you to get your chalk or sodium bicarbonate out for this style of beer. Infact some people will tell you not to use them, ever. Yes, even with dark beers but i'll leave that to thirstyboy to cover if he feels its necessary.
If youre really worried, some yeast nutrient wouldnt go astray as an added safety net for yeast health.