They are quite specific that they will endeavour to take your posting preferences into account, ultimately they will post you where you are operationally needed.
Like any job, certain postings (Melbourne, Sydney and Brisbane) are more sought-after than others, and competition for these postings may be a little more, shall we say, intense?, than postings to Launceston or Alice Springs.
If you only want to work in Sydney, tell them that up front, and you may get posted to Sydney, or they may not offer you employment.
If you tell them you are happy to work anywhere, they will believe that, but dont take up a posting somewhere else thinking you can easily transfer to Sydney.