Hi AP President,
A resignation letter is not confidential as of right. It must be claimed "confidential" (e.g. labelling the document confidential) and actually kept confidential (e.g. understanding between the sender and recipient that the document is only to be kept between the two and not to be disclosed to third persons otherwise legal consequences will attach).
Even publicising your resignation letter, whilst awkward, is not illegal. However, the publication should not contain your "personal information", meaning personal identifiers such as your address, date of birth etc. as this could be in breach of the privacy act.