Hi Mary,
Destroying mail is a federal offence. While there is obviously nothing you can do to get your mail back. You can inform the person that destroyed your mail that they have committed a federal offence and if you want to take it further I suppose you could inform the relevant federal authorities.
s471.6 of the Criminal Code Act 1995 (Cth) states:
"Damaging or destroying mail‑receptacles, articles or postal messages
(1) A person is guilty of an offence if:
(a) the person engages in conduct; and
(b) the person’s conduct causes damage to, or the destruction of:
(i) a mail‑receptacle; or
(ii) an article in the course of post (including an article that appears to have been lost or wrongly delivered by or on behalf of Australia Post or lost in the course of delivery to Australia Post); or
(iii) a postal message; and
(c) the person:
(i) intends that his or her conduct cause that damage; or
(ii) is reckless as to whether his or her conduct causes that damage.
Penalty: Imprisonment for 10 years."