Don't worry - unless you've been to Planning > Submit ICAO flight plan... and pressed the Submit button, chances are you haven't submitted a flight plan.
The AvPlan server actively looks for messages in the Australian ATC system that match the same combination of;
- aircraft callsign, and
- arrival airport, and
- departure airport
as a flight plan that's sitting open on one of your devices.
The probability is much higher for flying school or club aircraft that are having flight plans submitted of the round-trip type, e.g. from YSBK back to YSBK.
To prevent further messages like this, go though all of your devices and make sure that the flight plan has been put back in Stored Plans. This process sends a message to the server to stop listening out for the messages for you.
NOTE: Even if the app is shut down or in the background, the server is still dutifully on the lookout.
You can also optionally turn off all notifications like this by tapping Settings > Notifications and un-ticking Flight Plan Notifications.