Note that white-tails and redbacks have also migrated to NZ from Australia.
Also, from Wikipedia: "A katipō bite produces the toxic syndrome latrodectism; symptoms include extreme pain and, potentially, hypertension, seizure, or coma." Not exactly what I'd call charming.
> Though rare, the yellow-bellied sea snake is the most commonly seen sea snake in New Zealand waters, to the degree that the species can be considered indigenous to New Zealand and protected under the Wildlife Act 1953.
The katipo is native to New Zealand and the two spiders seem to been introduced by humans. The yellow bellied sea snake just floats over so often it's protected.
[1] https://en.wikipedia.org/wiki/Yellow-bellied_sea_snake#Distr...
I agree with everything you've said.
> Hornabrook, in his 1951 review of the early literature on katipo spider bites, found a total of 22 cases, including 2 deaths. Since 1951, there has been only one reported case of a katipo bite, involving severe myocarditis in a 22-year-old man, despite katipo spiders inhabiting coastal beach dunes around New Zealand.
[1] https://sciblogs.co.nz/griffins-gadgets/2012/03/09/bitten-by...
Bites were historically even more common when everyone had a dunny out the back.