The SBS On Demand app is not preinstalled on Chromecast with Google TV devices. It will need to be installed manually via the device's Apps Store.
You'll need to manually search for the app, rather than using the Universal Search or Voice Assistant tool.
You can install the SBS On Demand app by:
1. Select "Apps" in the top row of options. The "Apps" option is located next to: "You, TV Shows & Movies".
2. Under App Categories, select Entertainment and
3. The SBS On Demand app will appear in a list under "Popular Apps".
4. Select to Install
Once you've installed the app, you'll need to sign in. Click here for detailed, illustrated instructions on how to sign in.