Consumer New Zealand has raised concerns about the effects of smart metering technology being introduced by electricity companies.
The meters use a wireless modem to convey data, usually through a cellphone network, and can be read remotely.
Genesis Energy, the latest company to bring in advanced meter systems, will introduce them from 19 February.
Meridian Energy, Contact Energy and Mercury Energy have also begun supplying customers with smart meters.
They say this will put an end to estimated readings and visits from a meter reader and will help customers save energy.
However, Consumer New Zealand said the technology makes it possible to have tariffs based on peak times and off peak times, meaning there could be cheaper energy on summer days and more expensive power during winter nights.
It says that could lead to people cutting back on heating when prices are high, which would have a negative impact on their health and wellbeing.