I've received a reply from Australia Post.
Dear [Countessa],
Thank you for your enquiry submitted via our website and we apologise for the delayed response. We are currently receiving a higher than normal volume of email inquiries.
We have had complaints regarding this same issue from a few customers now. We have forwarded these issues onto our Online Support team, and they are looking into the matter. It is an unusual matter, as the link is working on our end, but not for some customers.
On behalf of Australia Post, please accept my apology for any inconvenience this matter may have caused you.
Yours sincerely,
Sarah T | Digital & Contact Centres - eServices | Australia Post
Online Services Consultant