Are you looking for a "deal" or just wanting to add an iWatch to your current plan?
If its a recent contract then you should be able to add an iWatch after the initial month. You'll need to contact optus directly.
Just an FYI on this one
From the 23rd of November, Apple Watches are now sold on the $15.00 and $25.00 24M Mobile Broadband plans (with a device repayment over 24 months in our Retail and our Tele-sales channel.
Customers will receive the following:
A physical SIM for use as a data SIM along with their Apple Watch
Number share is included as a $0.00 addon
Apple Watches can also be purchased outright, depending on stock availability.