Strong magnitude 5.5 earthquake strikes near Milford Sound, felt as far as lower North Island

A strong magnitude 5.5 earthquake has struck in Fiordland tonight and more than 6,800 people have reported feeling the tremor.

A stock photo of a recording of an earthquake's seismic waves

GeoNet says the quake at 10.35pm was centred 15 kilometres north of Milford Sound at a depth of 12 kilometres.

It says the earthquake was felt widely throughout the South Island and lower North Island.

A small number of people reported feeling extreme or severe shaking. About 1000 people felt it as strong or moderate while most felt it as light or weak movement.

Police and Fire and Emergency New Zealand told 1 NEWS they had not received any callouts related to the quake, about half an hour after it occurred.

Southland Civil Defence said the Milford Fire and Emergency team was activated and was investigating, but Civil Defence had no reports of any damage or injuries.

The quake was originally reported by GeoNet as 6.2 magnitude and later 6.3 before it was reviewed to 5.5.

"GNS have downgraded this to a 5.5, things are calming down a little in Milford - everyone a little shaken up, but OK," Civil Defence said.

GNS said it has located more than 10 aftershocks and this is normal following an event of this size.