Sounds like a suicide. Day after day in the sun without water will lead to that, if the means are available.
Drinking saltwater will also drive you insane...
I think that sounds like the simplest and most likely explanation.
There is a conspiracy theory that a Japanese sub assisted the Germans and possibly killed the Australian survivors but I think that if you would bother to kill survivors why leave the body in a float? I could see if the round was a machine gun or rifle round (suggesting they gunned them down as they floated in the water and sailed away) but it looks like they think it was a pistol round.