Wow! I have not heard about that! A 40m tal! Quandong!? Crikey! Tallest I've seen is about 10m. I guess they usually don't get that big in SA.

Must have been ideal conditions for it in your case and maybe it happened to be a particularly vigorous/fast-growing variant!? I have heard that it can be hard to get the seeds to germinate (sounds like it was working without troubles on your property!) I'd actually be kinda happy if it took over most of the grass at my place though I reckon! :)