There are two important issues to consider when thinking about importing any items from amazon.com that connect directly to mains electricity for operation or recharging:

Voltage & Frequency

North American electricity is 110 Volts Alternating Current at 60 Hertz compared to that in NZ which is 240 V AC at 50 Hz. Many electrical goods are multi-voltage (either automatic selecting or manually switchable) which means they can operate in NZ. The only way to determine the item's electrical requirements is to check its specifications. That information is often not listed on amazon.com so it maybe necessary to visit the item manufacturer's American web site to obtain that. If the specifications list something like "Input: 110–230 V 50/60 Hz" then it is multi-voltage and is compatible with NZ power. If the item is not multi-voltage then to use it in NZ you would need a Step-Down Transformer. Dick Smith Electronics (DSE) has a range of Step-Down Transformers for $50–400.

Some small computer related items operate or recharge using USB power (drawn from a computer's USB port and not directly from mains electricity). USB power is the same everywhere so those items will work in NZ too.

Plugs

If the item is multi-voltage the only complication is that it will most likely only come with a North American electrical plug. That can easily be overcome with a travel adaptor (about $15 NZD from DSE). Or if the power lead is detachable replace the whole lead with one for NZ (about $12 NZD from DSE), or it the lead isn't detachable (and only if you know what you are doing) cut the American plug off and wire-on a NZ plug (about $6 from DSE).

If you do buy a Step-Down Transformer then they include a North American power socket so a plug adaptor is not required for that case.