There is no free checked baggage allowance on Jetstar flights; you must pay for every bag to be checked in. You may purchase a weight allowance of 15 kg, 20 kg, 25 kg, 30kg, 35 kg, or 40 kg. The rate is dependent on when you buy it, as well as the exact route you are flying.
We always recommend you buy the checked baggage allowance ahead of time while making your booking (cheapest option) or by adding it to your booking on the Jetstar.com website afterward (more expensive). The most expensive is to pay for your checked baggage at the airport, as you will be charged a flat fee for the first 15 kg, plus a hefty per KG fee for every excess kilo you have packed.