The Kermadec Islands are the northernmost outpost of New Zealand, situated 650 km northeast of North Island, and 550 km southwest of Tonga. They are a Volcanic Island Arc, which is to say a group of islands that sits on a convergent plate margin (in this case the one where the Pacific Plate is being subducted beneath the Australian) and are formed entirely by volcanic activity. This volcanic activity occurs when rocks from the subducted plate melt as they are drawn into the earth's (hot) interior, then rise up through the overlying plate.
The Kermadec Islands.
The islands host four volcanoes, two of which, Raoul and Curtis, and a number of volcanic seamounts (marine volcanoes which do not poke above the surface) the most notable of which is Monowai Seamount, 120 m below the surface, and lies halfway between Raoul Island and Tonga - another island arc on the same convergent plate margin. To the south of the Kermadec Islands the South Kermadec Ridge Seamounts stretch most of the way to New Zealand; the most southerly, the Rumble IV Seamount, is only 150 km north of North Island, which itself lies on the Australian/Pacific margin.
The islands were settled by polynesians in the 14th century (and possibly separately in the 10th), but these settlements were long abandoned when the first Europeans arrived in the eighteenth century. Today only Raoul Island is inhabited, by a small group of meteorologists and conservationists.
At about 7.00 am on the 7th July (local time) a magnitude 7.6 earthquake was recorded 20 km beneath the Kermadec Islands. The earthquake was associated with the Kermadec Trench (i.e. the trench formed by the Pacific Plate being forced down beneath the Australian) rather than any volcanic activity. This triggered a Tsunami warning from the New Zealand Ministry of Civil Defence and Emergencty Management, but this was cancelled when no tsunami was forthcoming.
See also The Christchurch Earthquakes and Earthquakes on Sciency Thoughts YouTube.