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It isn't recommended that you use any such power converter for continuous use. They are intended for shorter-term use only. They are also much less efficient than power supplies and devices designed for the higher voltage to begin with.
Most devices with external power supplies can be solved by buying power supplies for the New Zealand voltage. This will likely be less expensive than a voltage converter would cost.
Some devices like hair dryers, curling irons, toasters, etc. draw large amounts of power and it will make more sense to sell your US equipment in the States, and buy new devices in New Zealand.
Upvote:0
If you have to convert voltage for long term use I would reccomend an isolating transformer with appropriate ratings. Note that most transformers have two ratings, a continuous rating and an intermittant rating.
I reccommend an isolating transformer because with an autotransformer if the "neutral" connection fails under light load you can see the input voltage on the output.
In the UK we get "Yellow brick" transformers used on construction sites which are 240V to 110V isolating transformers and are reasonablly affordable. For example https://www.screwfix.com/p/portable-transformer-with-2-output-sockets-3kva/84144 which has an intermittant rating of 3KVA and a continuous rating of 1.5KVA. No idea if you can get anything similar in NZ.
Honestly though it has to be a pretty special appliance for it to be worth it.