Tahi is available here in the US. Karaka Black is not. As far as trailing varieties go we have much better trailing blackberries than most of the world.
Karaka was trialed by the Royal Horticultural Society and found to be lacking for the backyard grower… but ymmv of course.
Riwaka is available here via Roger Ort who worked for Cornell Univ and must have obtained plant material via the best routes of that time. I have a few plants going but have no idea if its better than classic Boysenberry or not.
I discussed it a year ago but its still early to report.