Cool point, it seems it would have been a lot of work to retranslate all the English locales from scratch
Here is what happens in Transifex. When the TM detects 100% similarity matches automatically populates en_GB TM suggestions to the en_GB locale, en_NZ suggestions to the en_NZ locale, etc. This means that if en_AU suggestions are stored in your project’s TM instance, Translation Memory will automatically populate these translations to the en_AU language only.
There are a couple of workarounds for you to use, in order to manage this issue:
Once the translations of the en_GB language have been completed, you can download the TMX file from your project, edit it and replace en_GB with en_AU. Then upload the TMX file back to your project. You need to do the same for the other English locales as well.
Once the translations of the en_GB language have been completed, you can download the translation file and upload it to the other English languages of your project.
Hope this helps
Have a great day!