I have had no luck finding "gems" in the op shops. The stuff in the Salvation Army shop is mostly ex-Glassons and other retailers that specialise in the cheap 'n' cheerful. The stuff in the Red Cross shop is more of the same (though I did once find a Robyn Mathieson dress that was, unfortunately, a terrible colour for me).
And yet, bloggers across America seem to unearth vintage pillowcases, antique lacy bridal gowns and 60's retro fabrics all the time - and for very little.
Either I'm just terribly bad at snagging bargains (which, I'll admit, I am), or New Zealanders tend to hang onto their stuff for as long as possible - before selling it off on TradeMe. If our car-owning culture is anything to go by (i.e. we hang onto a car until the bodies fall apart from rust, and them try to flog it off to a student for a few hundred bucks), then this is an equally likely explanation.
I think I prefer the latter.