It would seem obvious to me that Australian English is closer to British English due to the historical events that led to English people living here. But it seems when differences occur that US English aligns more. Is this due to content from the US vs. Great Britain?
Australian English is quite like British-English. Around WW2, it was very similar, including terms like "pounds, shillings, tea(as in dinner), etc." still in common use. However, during the late 20th century, there is a rise of American English, being now used predominantly in movies, tv shows, etc.
Due to this influence of American English, Australian English is becoming increasingly like American English, as well as several other countries, like New Zealand, Canada, etc. (Not including British-English).
As far as slang is concerned, I've found that Australian and British are closer to each other than to American:
I'm not sure why this is. But it might be driven by the same force that makes the sports of Australia and Britain coincide a lot more than with the US (cricket & rugby).
(FYI I've lived in the UK and USA but never Australia, though at my secondary school there would be Australian exchange sports coaches who’d bark orders at me)