It's indefinite as long as the law doesn't change. But it would be easy for them to put a time limit on 'indefinite' RRVs, for example.
The safest way to insure your right to return to NZ is to become a New Zealand citizen.
Also remember that if you only have an RRV and not citizenship: - you can't have a New Zealand pbuttport - you have no special rights in Australia - any children you have born overseas will not be NZ citizens - your RRV could be cancelled if you got into trouble with the law, for example.
If you leave and return a few years later, you will probably have to wait out the 5 year residence period for NZ citizenship from scratch.
All that said, one of the requirements for NZ citizenship is that you plan to stay living in NZ. People do change their minds after being sworn in, but the bottom line is that if you decide to stay for citizenship, don't even think about any return to the UK until after you are sworn in as a NZ citizen.
Jeremy
-- This is not intended to be legal or professional advice in any jurisdiction
