Just some feedback for any other applications from Australia.
You need to provide tracking details of documents you sent to royal Mail tracking online
This will give you date time and name of receiver.
Then phone the UK passport office number which appears on their online site.
Wait wait and wait till you get through. After explaining the situation they will try and fob you off and tell you to keep waiting. Insist on speaking to someone else emphasize your calling from overseas. Pass on the tracking details and application number. I did this and my file was noted that my documents had been received but not confirmed yet. This will enable the application to proceed if the expiry date falls and will save my application fees. Sorry it’s long-winded but that’s the journey I’ve taken. Still waiting on acknowledgement but slowly getting there