The coroner investigates cause of death, and areas surrounding the death, including, where relevant, investigation into the death during an inquest. The coroner looks for ways the death may have been avoided, and can make recommendations about avoiding such a death in the future. The coroner in Australia has no power to convict any person- if the coroner believes they have found evidence of an indictable offence, it is then referred to the Department of Public Prosecutions.
WA inquests:
http://www.coronerscourt.wa.gov.au/W/what_is_an_inquest.aspx?uid=1393-9855-0518-7847
The coroner gets to look at the entire case- including the police investigation. The coroner can also call on suspects, or POIs, to give evidence, under oath, in the coroners court. There have been big cases in Australia where a coronial inquest has led to conviction- such as the Daniel Morcombe case, which was just so overwhelmingly big. A fresh review in a setting where witnesses and suspects are interviewed and cross examined was what was necessary to find those last few pieces that put the puzzle together, leading to the arrest and conviction of Daniel's murderer, and the discovery of Daniel's body.
Police will sometimes object to an inquest- sometimes because there is still an active, yet not public investigation into a particular suspect, or sometimes, a cynic might say, because they dont want the investigation placed under the spotlight.