I haven't really heard of aspen pellets. There might be some brand specifically for small animals, but you probably can't get it in larger sizes (pretty much anything for small animals is in small packages and more expensive). I have seem wheat straw pellets, but not sure if they are available in Australia.
Is there any reason you don't want pine? When made into pellets, they are heated and that removes the harmful oils. Other than a woody smell that goes away in a few day, they are safe to use and should not cause a problem.