Once a company has been delisted, you can go to Delisted.com,...

Once a company has been delisted, you can go to Delisted.com, pay a $150 fee to arrange a transfer of your holdings for $0, and write off your entire holdings immediately. It's a service has been around for many years and I haven't heard of the ATO rejecting a single claim.
https://www.delisted.com.au/sell-your-worthless-shares
I'm personally ideologically opposed to having to pay a fee to book a capital loss (as if a total capital loss isn't enough!), but the option is there.