Amphibian Throttle position
 

Anyone know why many amphibious aircraft have throttle controls on the overhead (twin otter is an example) ?

Maybe because that's where the engines are as they tend to be (are always ?) high wing types with the engines mounted above the flight deck.

............designed by an Aussie?!?! :}

From a mechanical point of view (easier & lighter cable routing) it makes sense on any high-wing multiengine craft. Also keeps cables out of the bilge.

Types I know of: Grumman amphibs, DHC-4/5/6

And the Avro York - not that you are likely to see one of those around any more!