Jetstar Return For Free! (edit: removed tomorrow reference for public sale as "tomorrow" is now today)
For your return flight, search for flights between 11 February to 3 April (only NZ routes available on these dates), 23 April to 3 July and 23 July to 27 September 2019. (Please check below as exact travel periods differ between destinations).
https://www.jetstar.com/au/en/deals/return-for-free
Edit: to find the free flights you seem to have to click the flight route you want from the sale page, then go right through to specific dates (the free flights do not show on the month summary but they are there if you click through)