Kia ora Te Rito, I am a local to the area you speak of. Were the plants bought from Kapiti Nursery? If so I can follow it up for you. I eco-source in the area too, and know of a few colonies of C. rugosa in the Otaki Forks, so it may well have come from there. That said, if you let me know the nursery I could answer this question with more accuracy as I know all of the nurseries owners well enough to confirm or deny.
Cheers Matt Ward