Just tried this with a trip i'm planning and the code worked fine
You must pay with a credit card that is enrolled in the citi rewards program
· Qualifying Bookings: This coupon code discount applies only to flight + hotel bookings (not a stand-alone hotel booking or a hotel booking in combination with any other product or offering such as or flight + hotel + car) made through Expedia.com.au/citi that are prepaid at the time of booking (subject to the restrictions set out in the Terms and Conditions below).
· Coupon code value: AUD $100 off
· Minimum Spend: AUD$1000 (minimum spend is the price of the travel reservation excluding any taxes, fees or additional costs). This spend must take place in one transaction.
· Redemption Period: until 11:59pm AEDT, 30 April 2018.
· Travel Period: The travel dates must be before 30 November 2018 (inclusive).
How to use your coupon code
· You must make your package booking with Expedia.com.au/citi
· During the booking process, on the payment page you will see the link ”Enter a coupon or promotion code” where you can enter the coupon code;
· Enter the coupon code in the relevant field and then click the “Apply” button. If your booking is eligible, the applicable discount will be subtracted from the price of the travel reservation before the application of any taxes, applicable fees or additional costs.
The key word is 'package'. I recommend that people research the price of booking flight and hotel separately for comparison.