Can you give us a bit more information, are you talking about Post Paid or Prepaid?
If your doing a Sim Replacement on a Post Paid service, you can contact Customer Service via live chat at http://yesopt.us/chat2us or use the Sim Replacement Tool and more information is located here http://www.optus.com.au/shop/support/answer/how-do-i-activate-or-replace-my-mobile-phone-sim-card?re...
You need to make sure you have a Blank Optus Sim from a Optus Store or sent out to you, the Store should be able to activate for you as well.
If its a Prepaid Service, you can use a new $2 starter kit as a replacement Sim and either contact customer service on 1300555002, or if you have access to the old Sim SMS the Word Menu to 9999 and follow the prompts to replace the Sim.
