English spelling has a lot to answer for. I'm currently in Sydney, Australia, leaving tomorrow to fly to Hobart, Tasmania, for my first-ever visit to that part of this excellent country. And I've just noticed, also for the first time ever, that the Australian nickname for Tasmania, which is Tassie, is pronounced with a [z], not an [s]. It figures, since the s in Tasmania is also pronounced [z]; but it doesn't fit the spelling Tassie, which, with its doubled –ss, ought to represent [s] (at least according to my intuitions about English spelling/pronunciation rules).
They could force a [z] pronunciation by spelling the nickname Tasie. Trouble is, that would also force a vowel change, so that it'd look as if it should be pronounced tay-zee. They could solve both problems by spelling the nickname Tazzie, but then it would no longer look like a shortened form of Tasmania. So instead they go for the spelling/pronunciation mismatch. Maybe it's deliberate, a trap for the uninformed tourist trying to sound like an insider. More likely it's just the typical (or stereotypical) Australian method of forming nicknames combined with an independent spirit that thumbs its metaphorical nose at sound/symbol conventions.
[Since a number of people in the comments below ask why I have not come in to address the matter (people may be aware that I spent something like a year and a half in Australia in bits and pieces during the time I worked with Rodney Huddleston on The Cambridge Grammar of the English Language [CGEL]), let me just say here that I addressed the matter myself in a passage that appeared in CGEL on page 1636. There is indeed a general rule of voicing intervocalic fricatives in embellished clippings. A clipping is a word formed from another word by leaving part of it out: ut
ility truck yields ute, the Australian for "pickup truck". An embellished clipping forms a clipping and then embellishes it with an ending: French letter ("condom") yields frenchie by addition of -ie, reg istration yields rego ("car registration") by addition of -o, and so on. When a fricative like -s- falls between the clipping and the embellishment, it is always voiced. The embellished clipping from position is possie, the one from Australian is Aussie, and they are all pronounced with [z]. Perhaps the most surprising is the embellished clipping formed from afternoon: it is pronounced [a:vo] (and usually spelled arvo, since Australian English is non-rhotic).