Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Townsville to Heidelberg as you normally would.
For a flight from Townsville to Heidelberg, you will be flying from Townsville. Townsville (TSV) is just 4.5 km from the centre of the city. Townsville is the only Townsville airport and it handles 0 outbound flights to Heidelberg per day. 2 airlines have flights to Heidelberg from Townsville on a regular basis.
The most popular airlines flying from Townsville to Heidelberg are Etihad Airways, Qantas Airways and Singapore Airlines. Something to consider prior to booking your flight.