Since Plato’s day (~400BC) the regulation of police forces has been on the public record as a vexed issue. It was probably a problem long before that.
The Canberra Times reports that more Australian Federal Police were caught being bad in the last reporting period (2007/8) than the one before that. 295 up from 92.
This is from a slightly smaller number of complaints.
It could be they’ve gotten better at exposing their own misconduct, or it could be there is more misconduct.