As JohnM has said, that's quite a complicated topic. It's a combination of climate, terroir, very poor soils (some of the vineyards such as Felton Road and Mount Difficulty the landscape looks more like the US badlands - no one wanted the place before the vintners came along) and , high UV giving thicker skins plus clone selection and vineyard management. Enthusiastic vintners with new modern techniques, many use whole bunch ferments which adds the green strong tannins.
I find the original ones were the best and some are now becoming a little too sweet. But others are far more knowledgeable and erudite than me!
Start here
timatkin.com
Central Otago – a serious producer of cheerful Pinot
www.jancisrobinson.com
www.pinotpalooza.com.au
Elsewhere this NZ Pinot produced by Giant Steps (Yarra Valley) is discussed.
www.giantstepswine.com.au
Recommended. A lovely wine grown from vineyards in the Pisa Range district. Good wine at a good price.