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Howdy. Hmm.. Hreflang is for telling google that there is the same article/page/post, just in different language. It doesn't really have much to do with actual rankings, unless you already have bunch of duplicate content. Since you said you are separating duplicate content, there shouldn't be any issues (btw, what exactly do you mean by "Separating duplicate content"?). As for rankings improvements - SEO, links, competitions research etc. Most likely the reason of ranking lower in AU due to AU competition. Or are the competitors who are outranking you in AU are the same as the ones which are not outranking you is US?

"If you use the hreflang, Google won't consider the alternate URLs as duplicate ever ;-)." I wish that was true. That would solve a lot of the .co.uk vs. .com problems that sites have where they want a different URL to show up in the SERPs without actually having to change their English content. But the reality is Google will consider it duplicate content even if you use hreflang. I have blogged about this in detail with examples: https://hreflang.org/hreflang-and-duplicate-content/