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Have you followed all the steps in the user guide? On the Telstra TV, go to Settings->System-> Advanced system settings->Device connect, and make sure "Enable Device connect" is ticked
Telstra TV uses Miracast for casting the screen of device. Since Android 4.1 Mircast was built into the operating system but since the introduction of chrome caste the settings for casting to Miricast devices has been hidden by google on later versions of the operating system.
You need to install a Miracast app. I use Screen cast. Open the app and follow the instructions for connecting to the Telstra TV.