In reply to:

The denizens of the Land of Oz (known in Zild as the West Island) used to have an expression, "raw prawn", to describe, I believe, idiots. Can anyone from Oz confirm or deny this? Or have you all just taken off to thrown another prawn on the barbie?

Yep, CapK, it relates to idiots, or more precisely to being naive or gullible. I've only ever heard it used as part of the expression "Don't come the raw prawn with me", meaning don't try to fool me.

And as you have hinted at in your post, and in a recent Fosters ad, we throw prawns on the barbie, not shrimps. In truth, I've never done either, but I've enjoyed the results.