This is a bundle discount deal!
You have to join both mobile and watch 24 months plan ($45+$10 /mth) and buy any iPhone 13 (any model), then you will get an Apple Watch 41mm (Cellular) for $0 or 45mm (Cellular) for $50
Example:
Phone: $1200 (iPhone 13 mini, 128GB)
mobile plan: $45 * 24 = $1080
watch plan: $10 * 24 = $240
So you spend $2520 to get a watch for free (value at $749)
If you have recently sign up for a SIM only plan for 24 or 36 months, talk to Optus, they could add this deal to your existing plan. Which mean u only need to pay for device payment and the $10 Watch plan for 24 months, and get an Apple Watch for free!
The page says 36 months.