Jetstar is having a 13th birthday sale and are offering the return flight for free on many domestic and international destinations. Various travel periods apply from July to March although peak travel periods are excluded.
Examples
Brisbane domestic from $69 return
Sydney domestic from $49 return
Melbourne domestic from $39 return
Perth domestic from $159 return
Adelaide domestic from $49 return
Darwin domestic from $129 return
Honolulu from $339 return (from Mel/Syd)
Tokyo from $299 return (from Cairns)
Ho Chi Minh City from $199 return (from Syd)
Singapore from $149 return (from Perth/Darwin)
Auckland from $159 return (from Gold Coast)
Bali from $129 return (from Darwin)
For your return flight, search for flights between 24 July to 27 September, 3 October to 25 December 2017 or 10 January to 29 March 2018.
wow, the new flight selection format sucks