Sorry to disappoint you, but it is very unlikely that you caught any species of rare turtle. First off, there are no native turtles on Norfolk Island or any of the islands in the vicinity (i.e. New Zealand, New Caldonia).
More than likely what you've caught is one of the following:
1) Red eared slider (Trachemys scripta elegans)
2) Malayan box turtle (Cuora amboinensis)
3) Chinese softshell (Pelodiscus sinensis)
All 3 of these comprise some of the most common turtle species on earth and all have been introduced to many areas of the world where they are not native. If you can repost the picture we can give you a positive ID.