Saturday night's massed electricity switch-off cut consumption markedly, says Transpower, the company with overall responsibility for New Zealand's electricity use.
New Zealand was the first of scores of countries turning off lights and appliances for an hour for the WWF-organised Earth Hour in order to highlight the effect of energy use on climate change.
Transpower says that at 8.30pm electricity demand dropped by 39 megawatts in the North Island and 24 megawatts in the South Island, a fall of 3.5%.
Transpower spokesperson Adele Fitzpatrick says Earth Hour caused a big cut-back in electricity demand, though not all of the reduction could be attributed to Earth Hour because factors such as the weather also played a part.
New Zealand organiser Dairne Poole predicted the hour-long event would be well supported across the country.
"Forty-four cities, towns and districts will be taking part. Iconic buildings like the Beehive, the Sky Tower in Auckland and the Octagon in Dunedin will put their lights off as well as hundreds of thousands of homes and business across the country," Ms Poole said.
While admitting that Earth Hour is a largely symbolic act, she says the aim is to send a message that people care about climate change.
Chatham Island began the world-wide event by switching off its diesel generators at 8.30pm.
Earth Hour was begun by environmental group WWF in Sydney in 2007 and has since spread to many countries.
Meanwhile, thousands of people were expected to attend a bicycle-powered concert in central Melbourne to mark Earth Hour.
"All this week a series of bikes have been linked up to dynamos that have been generating power which has gone into the grid," said Andrew Rouse from WWF Australia.
"All the cyclists madly cycling over the last week or so have generated enough power to more than offset the electricity use of the concert tonight," he said.
More than 8,000 people were expected at the concert to see performances by Nic Cester, Chris Cheney, Kram, Ash Naylor, Renee Geyer and Vika and Linda.