The ferrets in NZ that are feral ere not brought over by the ships. I
tend to believe the only ones still alive are the Hybrids.

"It has been claimed that New Zealand has the world's largest feral
population of ferret-polecat hybrids.[17] In 1877, farmers in New
Zealand demanded that ferrets be introduced into the country to
control the rabbit population, which was also introduced by humans.
Five ferrets were imported in 1879, and in 1882--1883, 32 shipments of
ferrets were made from London, totaling 1,217 animals. Only 678 landed,
and 198 were sent from Melbourne, Australia. On the voyage, the ferrets
were mated with the European polecat, creating a number of hybrids that
were capable of surviving in the wild. In 1884 and 1886, close to 4,000
ferrets and ferret hybrids, 3,099 weasels and 137 stoats were turned
loose.[18] Concern was raised that these animals would eventually prey
on indigenous wildlife once rabbit populations dropped, and this is
exactly what happened to New Zealand bird species which previously
had no mammalian predators."

[Posted in FML 7029]