I suspect this is something that will differ from university to university (in fact, I think usage can differ even between departments within one university).
As you perhaps know, once upon a time Australian universities followed the traditional British system (Lecturer -> Senior Lecturer -> Reader -> Professor), with a limit on the number of professors. In recent years there has been a move towards the US system (Assistant Prof. -> Associate Prof. -> Prof.), supposedly to avoid confusion or misunderstandings. However, this change is not universally popular: some people prefer the old titles and some resent the US-centric world view that is implied. There is also the issue that there is not a 1-to-1 match between the traditional titles and the US system, and so some people might see the rebranding as 'a demotion'.
Because this is essentially an HR matter, different universities have adopted different policies. Some use both systems simultaneously, perhaps depending on precise job role (e.g. to differentiate between teaching- and research-focussed staff). Some have essentially given people both titles and allowed them to choose which they use. Some have implemented the change within the promotion process - so that no new 'old titles' are created, but individuals retain the title they had unless/until they get a promotion, at which point they (effectively) sign a new contract with the new title. This may explain your observation that people remain as 'Reader' longer than they do as 'Associate Prof.': people who remain as Reader are those who are disinclined/unable to seek promotion.