I received an email from Sydney Opera House which although says for SOH members, the code still works when using from Etihad homepage.
Body of email:
Exclusive to Sydney Opera House members, Major Partner Etihad Airways is giving you 10% off your fare to Europe, the Middle East, Africa and the Americas. To take advantage, enter the Promotion Code SOHP216 on Etihad.com before 4 July, 2016.
Let me know if the title/description needs any corrections/updates.
*Terms and conditions apply. Offer ends 4 July 2016. Available for departures from Melbourne, Sydney, Brisbane and Perth for travel until 23 March 2017. Only valid on published return fares. Multi destination stop overs and open jaw bookings are excluded. Not valid for bookings of The Residence. Bookings must be made online and payment must be made during the booking process. Changes and cancellation of finalised bookings may incur additional charges. Offer is network wide. Some flights may be operated by code share partners. Visit etihad.com for full terms and conditions.