You do not need a transformer as such , you need a voltage converter that you plug into the wall socket which is 240 volts , the converter has one or more plug sockets clearly marked 120 volts and you plug your machine , phone charger or whatever else you bring from the states , into those sockets , surely they have those in NZ ? Many appliances etc come already able to be used with various voltages such as most computers , check your sewing machine , maybe it already has that conversion built in .Just ask at your electrical store for a Voltage converter , 240 volts to 120 volts .
colin