I doubt you contacted Roku if they asked for a credit card. Roku doesn't charge for support, so there would be no reason to ask for a credit card. Beware of scams!
You shouldn't have to make any changes to your network equipment except for the possibility of the Wi-Fi channel if you use one not supported in the US (I don't know what channels Australia may or may not use). You will have to create a Roku account at https://my.roku.com and it will ask for credit card information (supposedly to make users lives easier 🙄), but you can skip that step if you don't plan on making purchases or subscribing to anything through Roku.
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