Quite simple really, in the notification bar you will find something called "SmartThings". Fire up the AFL app and start playing audio from your favorite radio station. Then enable SmartThings and it will ask what device to cast to.
Except the primary function of SmartThings appears to be to "keep stopping". If you have a Galaxy Tab you will be used to this anyway. Every time this notification comes up it actually doesn't stop.
The licensing rights the AFL have put in place are insane. Intentionally breaking Chromecast, I can only listed to AFL broadcasts from radio stations from the AFL app and not the radio stations app, free for Telstra mobile customers and no one else.
Go the Pies!