We, and many other well-educated people, were instructed never to put hot food into the refrigerator. Then we read The Ultimate Cheesecake Book, by Joey Reynolds and Myra Chanin, in which one is instructed to put the cheesecake in the refrigerator as soon as it comes out of the oven. To say that we were flummoxed would be putting it mildly.We called the appliance manufacturers (who were surprisingly unhelpful, now that we think back on it), we almost wrote our congressman, before taking matters into our own hands and conducting an experiment.
We took one of those 3-lb, 350°F cheesecakes (delicious, by the way), whisked it from the oven to the top shelf of our smallest and wimpiest refrigerator (which was about half full), and measured the temperature of the refrigerator at both the top and bottom shelves every 15 minutes for four or five hours. Much to our surprise, the temperature hardly changed at all. On the top shelf, the temperature rose only about three degrees Fahrenheit and the bottom shelf didn't vary at all - and this with us opening the blasted door every 15 minutes. We found putting a hot cheesecake in the refrigerator to have less of an effect on the temperature than adding a new batch of groceries from the store.
The issue with allowing food to cool slowly to room temperature is that naughty bacteria are prone to thrive and multiply at temperatures between 40°F and 140°F (5°C and 60°C). And the US Department of Agriculture - concerned for your welfare even though you live in Australia - is deeply upset when foods are allowed to linger in that temperature range. The USDA will allow you to keep foods in that range for as long as two hours if you have to do something foolhardy, such as go on a picnic. But two hours is the limit. And, of course, they would prefer that foods not linger in that range at all (and you'll have to agree with the USDA that even if you put room-temperature food in the fridge, it's still going to take a while for the temperature of the food to fall below 40°F).
We would urge common sense on this issue. While we now bravely put fully-baked cheesecakes directly in the refrigerator, we don't crowd them up against milk other foods that go in and out of the fridge frequently. In fact, we don't butt them up against other foods at all. We would not put three gallons of hot stock in a refrigerator in any event. But an easy head-start for cooling something big and heavy or dense like stock is to put it in a pot in a sinkful of cold water, where it will cool to room temperature quite quickly. You can even change the water once or twice to speed the cooling. Smaller items, and foods particularly prone to spoiling, such as sauces and gravies, we would put directly into the refrigerator.