The voucher will be sent between 18 – 20 April 2016
Technically you can use the flight voucher right away and cancel the hotel you booked later if your circumstances change as most hotels have free cancellation up to 24 hours before check-in.
T&C do say that cancelled hotel bookings do not qualify for the flight voucher but can't see how they can enforce it since vouchers are sent by 20th April 2016, and cancellation can be processed by contacting the hotel directly.
With the $50 voucher, you could probably get yourself a free local flight with Jetstar Specials or via a Price match with Tiger/Virgin.
^Until 11:59PM (AEST) Tuesday 19 April 2016, book a stay at any one of over 150,000 hotels at jetstar.com costing AUD $200 or more and receive a $50 flight voucher. Flight vouchers are valid for 6 months from date of issue.
Once you've booked, keep an eye out for an email with your flight voucher details which will be issued using the name and email address of the customer who made the booking. We'll send the flight vouchers out between 18 – 20 April 2016. A maximum of one voucher per booking applies. Valid for any stay booked in a single transaction to the value of AUD $200 or more until 11:59PM (AEST) Tuesday 19 April 2016. A stay is one night or several consecutive nights spent at a single hotel even if the customer checks in and checks out more than once during that period. Cancelled bookings will not be eligible for this offer.