"therefore cannot update bank details"

Scammers manage to change the country of registration and other details on an account they've hijacked but it's very hard for the average member to do.

Verification  requirements vary  from site to site.

So unless you're verified and registered to sell in Australia , you won't receive any money.

https://www.ebay.com.au/help/selling/getting-paid/registering-seller?id=4792&srsltid=AfmBOoqLCWh6Ivm...

I recommend you cancel the transaction  for your sold item until you've sorted things out

https://www.ebay.com.au/help/selling/getting-paid/sellers-can-cancel-order?id=4136&srsltid=AfmBOopxd...