Price index: measures the change in price between time periods for a given set of goods and services. It summarises a set of prices for a variety of goods and services collected from a number of outlets.
Index reference period: the benchmark to which prices in other periods are compared (eg. if the index number in a later period is 1150, prices have increased by 15.0 percent since the index reference period). Prices for later periods can also be compared in similar fashion. The CPI has an index reference period of the June 2017 quarter (=1000).
Stats NZ reviews the CPI basket of goods and services every three years to ensure it remains relevant. This is done by surveying people to find out what they spend their money on.
The latest review was done in December 2017 (https://www.stats.govt.nz/reports/consumers-price-index-review-2017).
Impact of GST rise on the CPI: GST rose from 12.5 percent to 15 percent on 1 October 2010.
At URL provided, select Economic indicators > Consumers Price Index - CPI > CPI Level 3 Classes for New Zealand (Qrtly-Mar/Jun/Sep/Dec). Select all variables, then select desired download format. Figure.NZ calculated the percentage changes based on the indexes provided by the source.
The aim of the CPI is to measure price changes of the same sample of products at each outlet over time. When there is a change in the size or quality of any of the goods or services in the basket, Statistics New Zealand make an adjustment to ensure that the price change shown in the CPI is not affected by the change in size or quality.
The CPI is used to help set monetary policy and for monitoring economic performance. The government uses the CPI to adjust New Zealand Superannuation and unemployment benefit payments once a year, to help ensure that these payments maintain their purchasing power. Employers and employees use the CPI in wage negotiations.