DO NOT FLY WITH JETSTAR IF YOU HAVE A BIKE
As most people that fly with mountain bikes know they typically come in around 23kg if packed in a commercial bike bag.
Most airlines have at least a 23kg weight allowance and typically don't apply any oversize changes to sporting goods. I have flown overseas and intestate with Qantas, Virgin, Air New Zealand and Air Canada with a bike bag weighing between 23 and 24kg when packed and never once had an issue having it checked as my included bag. It has always without exception been carried under the flight's standard included luggage allowance
This all changed when I flew the first (and last time) with Jetstar (on the return leg of a qantas flight) Jetstar only allow 20kg (which if you use a good commercial bike bag and have mountain bike you're almost certainly not going to come in under) AND an oversize bag charge that isn't waived for sporting goods like most airlines. This charge along with their $15 PER Kg charge will add about $70 to your flight EACH WAY. They will not take into account that you flew out with their partner airline on the same booking with which can lead to some obvious confusion when you cant come back with the luggage you flew out with.
To top it of once they know you're at your weight limit the smell blood and really go in for the kill to make an extra buck. On our return flight the resorted to the utterly stupid level of weighing the wallets/purses you're carrying on board (even if they are not going in the carry on). While I acknowledge the whole "we need to weigh your wallet and purse" thing was probably just got someone having a bad day at check in it does go to underline the clear culture if making money from baggage charges however possible that must permeate through the airlines culture.
So if you're flying with a bike I'd strongly look at using an alternate airline