Unfortunately there are no Thetford service centres in NZ, the nearest is Australia (which I appreciate isnt of any use).
The model you provided appears to be an old model, which you may struggle to find Thetford Porta Poti original ports.
My suggestion would be to go to a local caravan and camping store, they often have generic parts for both Porta Poti and Cassette toilets.
Alternatively in your in NZ you may wish to contact the AU service centre and get them to dispatch the part to you via post, service centres in AU can be found here.
I have also checked online and cant see any parts designed to fit you model.
I just wanted to follow up and check if you required any further assistance, and request you rate the service received so I may resolve this case.