I agree - the T &Cs are reasonable. If you win you get a prize (eg "value" for a number of prizes is between $3,000 and $12,000 for amateur) and they get to use your winning image for 3 years. For non winning finalists the $100 voucher is a bit cheapskate (it's not costing Jetstar $100 is it!), and they also get to use the image for 3 years so you have at least some compensation, recognition of your ownership and copyright and there's a sunset clause.
What I found interesting is this clause:
Jetstar wrote:Entrants may use photo editing software to ensure they are entering the correct file size and the correct file format. No digital manipulation is permitted (i.e: cutting out elements, adding elements or undertaking any creative interpretations).
What is "digital manipulation"? Is it just big stuff like they say or is a curves adjustment or sharpening in that category? Same goes for "creative interpretations" - is making it black and white or adding a vignette or a selective blur creative for example?