Geoff, the problem is not that you can't leave - you can, the carnet is on the bike, not you. You just have to renew the carnet to keep it in effect, otherwise you will be in deep...
The point of the carnet is that if you do abandon or sell the BIKE in a country, the government gets the carnet money in lieu of you paying customs duty as you should have. They don't care about YOU.
When you finally return to the UK you can discharge the carnet no problem as long as you have matching entry and exit stamps. That's it.
You must keep the carnet document with you, don't leave it with the bike when you return, as you will have to give it to your carnet supplier in order to get it renewed at the end of the year. (In our experience Carnet must be renewed annually)
You may want to confirm wih your carnet company and NZ customs that a stay of one year in NZ won't be a problem. It may, thats a long time.
Note that you don't actually need a carnet for NZ, and in your case you may not want to use one.
From the Shipping page in Tripplanning:
"At the customs office - a five minutes walk from the air terminal at Christchurch - the custom's official will ask for the value of the bike, then take 12.5% (NZ GST) of the amount and that is the amount that will be asked for and kept in an account that is established and held until you depart."
Contact NZ customs for clarification, they're easy to deal with, and it's not like they were one of the "more difficult" countries.
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