[mylesp]

In Australia from my experience "not bad" = "good", "pretty good" = "amazing", "bit shit" = "really shit".

I don't think its as much that everything positive is just a non-negative, but that everything (especially emotions) is shifted towards the medium. Maybe it comes from a desire to not be abrasive and always soften everything, but I'm not sure.

[PlunderBunny]

Related also to the way Australians and New Zealanders use understatement for humorous effect, e.g. that last example you gave could be used to describe any condition up-to the point of death.