[AZKen]

Maybe you have a 2500 series. You are feeding us info very slowly and incomplete. You make statements without backup or data to support. I don't know anything about NZ requirements.....but we need data for this type of hard to find item and be sure we are looking for the right truck series. Trying to be sure non standard modifications have not been made. I.E. donor chassis, donor differential/axle, wrong vin tag due to swapping, etc.
1. What is actually wrong with the drums to make them NZ non compliant?
2. You say ID of rear is 12". What is ID of front. What is the inside width? 2 1/2? From your title, are we to assume you only need rear drums?
3. Is NZ going to take drums off and inspect/measure?
4. Are you saying NZ does not allow drums to ever be turned???????? Is there no appreciation for classic trucks and the lack of parts?...and enormous shipping costs.
5. If the minimum wall thickness of the drum, which is stamped on it, is not violated, the drums don't need turned/replaced. No matter if grooved or not, Shoes will comply. US DMV does not inspect old trucks and they sure don't take off the drums.
6. You will probably have a really hard time finding new drums. It seems the GMC Series 2500 has 12" x 2 1/2" shoes.
7. What is the VIN of your truck?

There are several older posts on here discussing drums and part numbers. In the end you may have to buy used drums in USA.

Arcing shoes and turning drums was always a scam to make easy money. Also...Now-a-days parts are pretty accurate. Shoes last way longer.