Actually, in QLD it's the Office of the Director of Public Prosecutions, not the police who have the discretion to lay charges. The role of the police is specifically to investigate, gather evidence and present to the DPP.
DPP then decides whether to proceed based on the Director's Guidelines; specifically Section 4:
4. THE DECISION TO PROSECUTE
The prosecution process should be initiated or continued wherever it appears to be in the public interest. That is the prosecution policy of the prosecuting authorities in this country and in England and Wales. If it is not in the interests of the public that a prosecution should be initiated or continued then it should not be pursued. The scarce resources available for prosecution should be used to pursue, with appropriate vigour, cases worthy of prosecution and not wasted pursuing inappropriate cases.
It is a two tiered test:-
(i) is there sufficient evidence?; and
(ii) does the public interest require a prosecution?
So presumably they felt there was not only sufficient evidence to convict, but that it satisfied the "public interest" criteria.
For:
5 (iv) Sexual Offences - Sexual offences such as rape or attempted rape are a gross personal violation and are serious offences. Similarly, sexual offences upon children should always be regarded seriously. Where there is sufficient reliable evidence to warrant a prosecution, there will seldom be any doubt that the prosecution is in the public interest.
Against:
5 (ii) Aged or Infirm Offenders - Prosecuting authorities are reluctant to prosecute the older or more infirm offender unless there is a real risk of repetition or the offence is so serious that it is impossible to overlook it. In general, proceedings should not be instituted or continued where the nature of the offence is such that, considering the offender, a Court is likely to impose only a nominal penalty. When the defence suggests that the accused’s health will be detrimentally affected by standing trial, medical reports should be obtained from the defence and, if necessary, arrangements should be sought for an independent medical examination.