Hello Emmaaa-,
If you buy an Apple Watch with cellular, you can use it independently of your iPhone as the cellular service is activated on the watch directly.
This resource covers that process: Use Apple Watch with a cellular network – Apple Support
You may also find more helpful information about using your Apple Watch without an iPhone nearby here: Use your Apple Watch without your iPhone nearby - Apple Support
Also, you don't need to purchase it from Telstra, just refer to this guide before purchasing: Set up cellular on Apple Watch - Apple Support
If you still have questions, reach out to Apple Support directly, as this may require closer guidance: Get Support
Cheers!