Optus has confirmed its plans to shut down its low-cost subsidiary, with the Virgin Mobile brand closing down entirely by 2020
If you're an existing Virgin Mobile customer, you should be contacted by Optus to discuss your options directly, and you don't have to panic about losing mobile service. This is largely a brand play rather than a network closure, but it's always wise to know what your options are in these circumstances.
No doubt Optus would prefer to keep you as a customer, but depending on your contract status, you've actually got plenty of flexibility. That's true even if you want to stay on the Optus network because it's by far the most widely used network by Mobile Virtual Network Operators (MVNOs) in Australia.