I think it's a little too small to be the mouthpiece for an instrument though admittedly I don't know much about those types of instruments... will take another pic tomorrow with a coin or something for comparison, but I can tell you the total length is around 20mm, the large mouth is around 10mm and small end around 5mm.
I'm leaning towards the mouthpiece/tip of a cigarette hold, much like in the image golddiggerart posted above. Dimensions look almost identical.
Also just based on where I found it, deep in the rainforest upstream in a small creek. There's nothing further upstream, it just goes deeper into the rainforest and the creek eventually peters out... so the only people going up that far into the creek have to be prospectors/explorers, likely from quite awhile back. I wonder did the old timer Aussies use these cigarette holders back in the day? Or more likely Chinese?