Unfortunately the option to reprint the label only remains valid for 24 hours, after that, the option is gone, so that's why you're now getting the error.

I'm not sure if there's any way for you to be able to acess the original labels and print them (eg by phoning Aus Post to see if they can assist). In most cases they usually suggest to purchase new labels, and cancel the old ones (which I find takes an average of two weeks, to get the refund, I mean). 

I know this won't help now, but it might help in the future - every time I generate postage labels online, I save a copy of the PDF (I change the file name to the date the labels were generated, plus some other identifying info, so I can quickly locate it in my folder), it's mostly so I have a copy if I need if for proof of post, but it's saved me a few times from having to buy new labels when something's gone wrong and I couldn't print from the original PDF that's generated.