How would you prevent someone boarding from the San Francisco to Narita, staying a couple of days in Tokio and flying to New Zealand? With a forced quarantine in each stopover?
You'd see that they'd come from SFO originally and not allow them to board the NZ flight at Narita. Or at least require x weeks quarantine and two negative Covid19 tests, or whatever conditions you want to impose, before they can continue to NZ.
You seen to be ignoring the creativity of some people. Someone could buy a ticket on Delta Airlines SFO - NRT and a separate ticket for next day on Air NZ NRT - AKL. Unless the Japanese keep tight control of entries and exits on our behalf we would have to make sure NZ Immigration systems could track a passenger's travel even on different routes before jumping on the last leg of the trip.
Do you think any country's systems are there yet?
The way around would to only allow people on non-NZ passports to fly in if they have evidence of a 14 day quarantine on the last leg of the trip before flying to NZ and even so I wouldn't trust an American coming to NZ to be virus-free.