SpookyAwol: Theres a difference between acceptable and legal.
As mentioned before - you need to carry your original licence as a legal document
A photocopy may be acceptable as it at least identifies you and gives the cop something to work with. However if you chose to carry no form of ID at all, they have the right to detain you to prove identity.
Smart devices are now being used by police that scan your licence to provide an immediate ID. If you dont have your licence, prepare for a longer wait.
These devices don't yet have the capability to pull photo's across so the roadside cop cannot yet see if the person driving is indeed who they say they are.*
Aside from this, police do not have general access to the LTNZ drivers licence photo database, so are only able to view (at a police station) photo's of people who have been arrested. If a driver has never been arrested before, police cannot see a photo of him/her, except on their drivers licence.
EDIT: *mentioned this in light of a combination of mattwnz's and SpookyAwol's post