There seems to be plenty on places within Australia and internationally that do print on demand for shirts, jumpers and tank tops however I cant find any that do print on demand for shorts or tracksuit pants. I have reached out to a few who have said that they don't stock these items.
Does anyone know of any Australian ones preferably that do print on demand for tracksuit pants and shorts?
Actually, Printful does offer custom tracksuit bottoms and shorts. Here's the link to them in the product catalog. They're not Australia-based, though, but they do have two partner fulfillment facilities there.
Hope this helps!