See https://www.rbnz.govt.nz/statistics/b13 for a full description.
The historical data is split across multiple files at source. We download all of them and merge together into a single data frame, but note that unlike with most of the other series available here it is not a simple matter of rbind-ing all the data together. The composition of the TWI changed in December 2014 but there were some components that were present both before and after, for example Australia. These are returned as separate fields in the output rather than combining them into a single entry.
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