Hi Debbie

If Catherine was adopted then it would have been an informal arrangement - legal adoption did not start in NZ until 1880-81.
There may have been a fostering arrangement but this is usually between the two parties and at that time there would have been no legal or government involvement - so no file. In this situation children's surnames were often changed so as to fit in with the new family but this was not a legal name change.

Best wishes
Elli