In New Zealand there is no official definition of what is rare. That is because we have no government or health system policy or programmes which are specific to this category of disorders.
However, there is widespread use of the phrase around the world and some definitions in various countries which provide useful guidance. RDNZ generally follows the European Union policy which defines a disease or disorder as rare when it affects less than 1 in 2000. This definition is in their Orphan Drug Regulation 141/2000.
While they are individually rare, collectively these uncommon health conditions affect up to 6% of the whole population. This estimate is based on a number of surveys and reports from the US and the EU over recent decades.