GeoNet is reporting a 'strong' 6.3 earthquake has struck in the lower South Island.

The quake, measured a depth of just 5km, hit 20km northwest of Milford Sound at 10:35pm.

Nearly 7000 people have reported feeling the quake online.

Initially reported as a 5.5 magnitude, it has since been revised up.

GeoNet has measured at least five aftershocks measuring over 2.6 in the aftermath of the jolt.

M5.5 quake causing strong shaking near Milford Sound https://t.co/bnvXGwUjaa

— GeoNet (@geonet) August 12, 2019