I can replicate it in a VM; English-US and English-UK work fine, English-AU does not.
However, this is also true for other languages: German(Germany) and French(France) are shown in VoiceAttacks list, whereas Danish, Italian and Dutch are not.
The "Choose the language you speak with your device" dropdown excludes some of the same languages VoiceAttack does, yet it does show English(Australia) and Italian(Italy).
Looking at the optional features list, Danish and Dutch don't actually include a speech recognition engine, so them not showing is normal. However, Italian and English(AU) do have speech recognition installed.
Here, an important detail is that Italian is described only as "Cortana for Italian", where as German for example is described as "Cortana and speech recognition for German".
While English(AU) does include the "and speech recognition" in the description, looking at the file size I suspect that may be erroneous; German speech recognition has a filesize of 82.9MB, and all languages that VoiceAttack does recognize are larger than 70MB, yet English(AU) and Italian are only 7.6MB and 7.32MB, respectively.
So, it appears the issue may be on Microsoft's end, as there is no speech engine downloaded that VoiceAttack can use for your language.