how likely is it to get a posting to a major like Melb/Syd/Bris?
Varies, most (all) of the towers would like an extra person on the roster. Individual preferences come into play as does the needs of the company, plus there is (often) an attempt to match the person to the Tower that will give them the best chance of getting rated. Sydney Tower has a few "new hires" (ab-initios), well, not so new now.