It is FREE.
With this Android application, you can easily access to official Opal website and logon and logoff.
Not record your user name and password, all you input is directly going to Official Opal website(https://m.opal.com.au).
In this App, you can see the balance of your Opal Card, the fares deducted, the Journeys and the Trips, All the data is synced with Official Opal website.
Safe, Simple, Satisfied