They know when you're out and when you go to bed.

The Office of the Privacy Commissioner yesterday fired a warning shot at power companies over the huge amounts of private details that smart meters collect every day.

In a memo, the office said such meters, which are now installed in more than a million New Zealand homes and businesses, could take electricity readings detailed enough to determine whether customers were using high-energy appliances, such as ovens or heaters, and when they had left the house.

But power companies said it was not possible to gather that level of detail, and all private electricity information was stored securely.

The office had received a growing number of complaints concerning smart meters, with customers claiming the collection of their real-time electricity readings amounted to a breach of privacy.

While none of complaints had been upheld, the office said the readings undoubtedly contained private information, and power companies needed to be more upfront about their handling of the torrent of personal information gathered by smart meters.

Power companies would also need to be more careful about who they shared the information with, to avoid breaching people's privacy.

"Even though power companies can collect more ... information about power usage, it doesn't necessarily follow they can use that information for any purpose they choose," the memo said.

"They also need to have strong security standards to ensure information is transmitted safely online."

The memo also raised concerns about how personal information gathered by smart meters could be used by the rising number smart appliances, which could connect to each other through the meters.

Smart appliances are already raising privacy concerns, with Samsung revealing this week that its smart televisions could secretly record and transmit the contents of private conversations.

Overseas, smart meters have faced a small, but loud, customer resistance movement. Some have raised concerns that information could be hacked or sold to thieves, used by insurance companies to deny claims, and become discoverable in court cases, much like phone records.

Former Greens MP Sue Kedgley last year switched power companies to avoid a new smart meter. She said she was concerned about privacy and electromagnetic radiation.

"I just don't like the idea of someone monitoring my electricity use every half an hour. It's quite intrusive."

Smart meters have been introduced in New Zealand over the past few years and now read the electricity from about 1.2 million homes and businesses, outnumbering the old meters.

By reading electricity use twice an hour, the theory is that customers should be billed only for the power they use, and energy companies are able to use the stream of almost real-time information to plan supply more accurately.

Electricity and Gas Complaints Commissioner Judi Jones said smart meters did give power companies a lot of personal information that could be used to work out household routines, such as when customers went to bed or when they left the house.

"Over the course of the day, you can say what is being used or not. You can see when the kids come home and turn on the television."

However, there was nothing wrong with collecting the information, providing customers were informed and power companies treated it sensitively and securely, she said. She had received three complaints raising concern about privacy, among other issues, and smart meters, none of which were upheld.

Power companies who responded to Fairfax Media yesterday said information from smart meters was kept securely and only sent to third parties and then only when required by law, as with phone records.

Contact Energy said that, despite claims to the contrary, it was not possible to monitor individual appliances, because power use was recorded only once every half an hour.

SMART METERS

What: A catch-all term for a new generation of electricity meters that can read your power usage accurately and transmit that information directly to your power company. The readings on old analogue meters needed to be physically checked by the power company, which meant they often estimated your monthly electricity bill and corrected it later.

Why: Through more accurate reading, you pay only for what electricity you use, and don't get stuck with a big bill when your power company guesses poorly. For power companies, it helps make supply more predictable and saves on the cost of checking meters physically.

Where: Pretty much everywhere. There are more than 1.2 million meters already installed in New Zealand, outnumbering the 900,000 remaining analogue meters.