There's some bafflement at mystery lights which lit up Wellington's sky as the 7.5 magitude earthquake struck overnight.

Videos of the colourful lights have been watched by millions on social media.

Amid much speculation, the lights appear to be part of a rare phenomenon labelled by National Geographic experts as "earthquake lights", caused by the electronic properties of certain rocks when under tectonic stress.

"Earthquake lights" can come in several different forms and stay in the sky for minutes.