Optus
In some cases, you may be able to request a deletion of your personal information held by Optus or one of its brands.
Spoiler
This isn't true. Optus don't allow you to delete your details.
What to expect
If you want Optus to delete your details, first, ensure you haven't been a customer for at least 2 years. Data retention laws require Optus to hold your data for at least this long.
Next, you have to make a genuine attempt at asking someone to "delete your details". Keep notes of who you spoke to and when.
The Optus "live chat" people will tell you "it is possible" (which it's not), you just have to go into a store. When I did this, the manager showed me his system and demonstrated that there was "no way" for him to delete my details.
At this point, Optus should advise you there's "nothing more they can do to help you delete your details from their system". Without any legal justification for holding your details, you now how a good reason to contact the Telecommunication Ombudsman.
The Ombudsman will ask you to provide Optus with 10 days to provide you with a resolution to the problem. For me, it took 7 days before I received a call.
You'll likely speak to an "Expert from Customer Resolutions". They aren't well informed on privacy laws and are hazy on what legal reasons they have to hold ex-customer data beyond two years. They will tell you it's related to telecommunication and taxation laws but won't be able to provide you with anything specific.
Beyond data retention laws, the Telecommunication (Interception and Access) Act 1979 does provide some reference to Optus's requirements to hold your data in Section 187C. However, once again, this is limited to "2 years after it came into existence".
The outcome
Currently, my case still resides with the Ombudsman. The hope however is that Optus recognise that their duty of care to their ex-customers obliges them to provide a fast and easy way to have their details removed from their system.