OP here.

Some more information here (not my preferred source, but oh well): https://www.news.com.au/technology/online/hacking/up-to-9-mi...

It seems around 2.8m have had 'all' data stolen (including ID, address, etc), and around 7m 'just' names, DoB and numbers/e-mail addresses.

Apparently Optus is working on sending personalised details to customers.

What a monumental stuff up.