You'd think there'd be a right of appeal, otherwise - as much as it would be stupid for the AEC to be unscrupulous - they could just write a whole bunch of fines willy-nilly?
Now who has the burden of proof and how one proves that they really were voting is a different thing... Stat Dec? With a witness? More like, if one was there when the rolls were marked (you hand over your ID), how is it possible they did not check off the name, which is what they must do before handing over the ballot papers?
How does it work? Do they have a bogus address, BPAY setup or URL where you can send a cheque / pay the fine?