Calcium absorbs excess water the way limestone perks water in a field. So when you take a calcium pill, it just sucks up the excess water in your digestive tract and that's what keeps you from having diarrhea. Me, I have excess bile in my intestines and while the calcium helps some, it doesn't do as well as medicine I take just for absorbing bile. And some people have a transit time so rapid that calcium and all just rush through without getting much time to work. So depending on what's causing your diarrhea, the calcium may not be a cure. But it seems that most people do get some help from it. Also, calcium can help heal your intestines if they are irritated or inflamed by viruses, bad bacteria, excess acid, stress, allergic reactions, etc. I did get my guts tore up last summer from a combination, I think, of allergic reaction to food and excess bile and I hurt all of the time and had diarrhea that would not stop and the calcium stopped the pain and got my diarrhea back down to my usual levels so my bile medicine alone was able to handle it again.
The Caltrate 600 w/ Vitamin D is the recommended brand because it is the right type of Calcium (carbonate, I think) and it has a minimum of magnesium. You have to have magnesium involved in order to absorb the calcium into your bones, but it also acts as a laxative and counteracts the constipating effect. Obviously most people who are on calcium for their bones don't want to be constipated, so most supplements have plenty of magnesium in them in order to combat the constipation. This particular make and model, however, has the lowest amount of magnesium of the brands out there, which means you get maximum water absorbtion.
I think you said you were in Australia? This brand may not be available down there or may go by a different name. Just look through the different bottles of calcium and find one that's calcium carbonate and get the one that has the smallest amount of magnesium in it. You can always ask the chemist to help you find the most constipating form of calcium, lol. I'm sure he's had plenty of people come in and complain about one brand or another.