If I understand the question correctly that message is saying you have consumed all of your shared data from both your WiFi and your mobile. You should also get a message saying a $10, 1GB topup has been added if all your data is used.
If you check under data usage on your phone you should know how much the phone has used. I guess the rest has been used by the WiFi if that's the other data pooled device.
Edit: The Optus account portal should show what device has used what data. This information may also be available in the Optus mobile app.