well I think it could fit over a larger diaper, but if you really want that roomy feel, you will either need to get a size bigger then you, which will mean all of it will be baggy, or something more designed for a lot of physical work, which can be quite uncomfortable. It's been a while since I looked around the gold coast or Brisbane, but I don't remember ever seeing anything that was comfortable to wear yes was extremely lose around the crotch.
Your best best may just be a normal pair of overalls designed for normal wear, and hoping it's got room enough for a bigger diaper, without really asking my friend, I don't know how baggy it is.