A few ideas:
Is there a celebrity he despises? Decorate the whole office in pictures of that person (or people).
Does he have issues with people touching his stuff? Take pictures of everyone touching everything in his office.
Rearrange everything so that it’s similar to what it was but just a bit off so it will annoy him because things aren’t quite where they should be.
You guys play games at the office – does he have any saved games? Maybe… remove them? Or move them somewhere else. i’m not that evil.
Does Australia have any rivalries with other countries? Do they dislike New Zealand, for example? I honestly have no idea. But if they do, decorate the office with stuff from that other country.
Is there a scent he really dislikes? Put some in random hiding places he might have difficulty finding.
Hide his cereal.
Jeremy, if any of my ideas are used please know that I am terribly sorry and please don’t delete my account.