Rather a worry hearing this though it does not mean that the hackers can access private information UNLESS people are taken in by the websites the hackers have managed to create purporting to be mygov and medicare.
This advice from :https://www.staysmartonline.gov.au/alert-service/beware-fake-medicare-email
Do not click on links in emails or text messages claiming to be from myGov or Medicare. myGov will never send you a text, email or attachment with hyperlinks or web addresses.
Don’t open messages if you don’t know the sender, or if you’re not expecting them.
Be suspicious of messages that aren’t addressed directly to you, or don’t use your correct name.
Login to your official myGov account by typing the web address into your browser, to check your inbox for any legitimate emails from Medicare.
You can also contact the organisation separately to check if they have sent the message