The quake hit 193 kilometres (120 miles) north east of the main Raoul Island at 5:23 p.m. (0523 GMT), the United States Geological Survey said.
There were no initial reports of casualties or serious damage.
The quake struck 746 kilometres south west of Nuku'alofa, the capital of Tonga, at a depth of 10 kilometres.
The Kermadecs, New Zealand's largest marine reserve, are uninhabited apart from conservation officers and volunteers on Raoul Island, according to the New Zealand Department of Conservation website.

Copyright AFP (Agence France-Presse), 2006