Now, will you look at that - a surprise fry!

So, he basically can't be older than 2-3 weeks, right?
How cold does it get out there at the moment? Wait, Australia has summer right now, so no cold? He definetely can survive out there, the plants and small particles in the water provide him with enough nutrients to munch on, and he is a single fry, so there is plenty just for him alone.
The question is, would you rather have him someplace where you can control better how he eats, and even get him a little filtration? How big is that tub he is in now? Maybe if you stick a small sponge filter in there, it'll make him really happy.

Or you can always get that 5 gl tank and stick a sponge filter in there, preferably one that has been in an established tank already, so its cycled. The move shouldn't stress the fry too much, since you would be using water from his tub right now, without adding any other water at the moment.
Baby, baby, baby.....