Found this coupon code advertised on their website while I was researching. Must be used on the Australian hotels.com website and for the first 3,449 customers only.
Coupon code valid until 31 Dec 2016 for travel up to 31 March 2017.
Full terms & conditions can be found on the website but I have copied and pasted the main parts:
Use this coupon to get A$40.00 off the price of your booking at a participating Price Match Guarantee hotel when you stay between 1 and 28 nights and spend a minimum of A$360.00 for your entire stay.
You must pay for your stay when you make the booking. The discount only applies to the first room in the booking. You’ll need to pay the full price for any other rooms. The discount doesn’t apply to any taxes, fees or additional costs.
To use this coupon, you must be over 18 years old and resident in Australia. You may only use this coupon for bookings made between 12:01 am MT on 01/10/2015 and 11:59pm MT on 31/12/2016 on the Australian version of Hotels.com for a stay to be completed between 01/10/2015 and 31/03/2017. Bookings are subject to availability and the hotel’s terms and conditions.
Go to http://au.hotels.com/page/au-hotel-exclusions/?pos=HCOM_AU&l… for a list of non-participating hotels. Participating hotels are subject to change at any time.
You can only use this coupon for a maximum of five bookings and each time it must be used in full. After those five bookings you won’t be able to use this coupon again, even if you cancel one or more of those bookings.
This coupon is only valid for the first 3449 bookings made by all customers using this coupon. You’ll be able to check that it’s still valid before completing a booking. There’s a limit of one coupon per booking.
This coupon can be combined with discounted prices on Hotels.com. It cannot however be used in conjunction with any other promotion on Hotels.com, whether offered by a third party or otherwise.
Also worth considering the COF12 promo (12% off) code if you're spending more and booking with MasterCard.