Apparently the National Archives has an online copy of the Australian Government file on Fred Valentich's disappearance. It has 317 pages in the file and includes reports from a number of government agencies including the Department of Transport, BASI (Bureau of Air Safety Investigation), Royal Australian Navy, RAAF and others.
I've only read the first 20 or so pages of the file so haven't had a chance to look at it in detail. Here's the link to the Government file on Valentich's disappearance, file number
V116/783/1047.
Below is a screen capture of one of the reports from the file, from the RAN Research Lab, on the wreckage found washed up on the coast of Flinders Island in 1983 which is believed to be a cowl flap from a Cessna C182.
