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I can watch the games fine on iPhone, iPad and PC but when I try to cast it to Chromecast i receive "Session not authenticated" and cannot stream to the TV.
Chromecast doesn't do it right. It uses Google DNS, or something. These services are only warranted for direct viewing on a tablet, and don't even warrant wifi access.
One way around, is an Android TV box, if you can first prove to yourself it works over wifi.
On ebay you need to search for Android 4K Kodi 2GB S905 Mali 450 or better. Avoid buying directly from "China". The fake northern terroritory sellers can deliver OK. Don't let them swap your order for something inferior.
Optus Sport is compatible with Chromecast so it should work. Troubleshooting is limited though, have you tried restarting your Chromecast/app? If the issue persists, I would raise it directly with TV & Content here → Report a problem with TV or Content