Trades workers are taken from the census.
Their 'origin' region is based on where the address where the worker answered the census.
Their 'destination' region is derived from primary region found in the address notification table in 2018. The address notification table summarizes all addresses reported to government agencies, e.g. if you visit the hospital and update your address while there this address will pass through Ministry of Health records through to StatsNZ and be recorded in the address notification table.