For each site, NIWA merged temperature records from local sites to form a long time series. The data are adjusted for climatic differences between sites and changes in exposure or instrumentation at the same site. This prevents the introduction of significant biases with time.
This data was also converted to global mean annual temperature anomalies (difference from normal), based on the same period we use for the New Zealand data (1981–2010)
NIWA's 'seven-station' temperature series uses temperature measurements from seven 'climate stations' (NIWA). The sites offer a representative geographical spread with reliable records dating back to the early 1900s.