Based on observations of Cabernet Sauvignon phenology, cropping and fruit characteristics at six vineyard sites in Hawke's Bay (New Zealand) studied over three seasons, a numerical model is proposed to characterise environmental conditions of a vineyard site. The proposed model is based on air temperature in October and January, seasonal rainfall, rooting depth, gravel percentage and clay-to-silt ratio in topsoil. The ‘Site Index’ (SI) calculated from these variables was significantly correlated with soil temperature and volumetric soil moisture content, themselves closely linked with clay-to-silt ratio, air temperature and rainfall. Vegetative growth, canopy characteristics, precocity of veraison, total anthocyanins, TSS and malic acid concentration in grapes, as well as wine sensory score, were all significantly correlated with the SI values at six sites and over two seasons. Over the same period, correlation of SI with several viticultural variables was generally stronger than five existing climatic indices for viticulture calculated for these sites. The proposed SI index appears to have potential use for vineyard zoning and site selection and evaluation.