The quake may have been felt in Balclutha, Gore, Invercargill, Lumsden, Roxburgh, Te Anau and Tuatapere, GeoNet said.
The quake occurred in a seismically active area where there have been some well-documented big shakes recently.
It happened near a plate boundary where the Australian plate dives below the Pacific plate - the opposite way to how the two plates subduct in the North Island.
Called the Puysegur Trench, this boundary stretches for more than 800km south from the South Island, to a point in the wild and windy Southern Ocean, around 400km west of the Auckland Islands.
Today's event followed a 6.4 quake in July, that struck 190km southwest of the subantarctic Snares Islands and was also felt by people around the South Island.
In 2009, the 7.8 Fiordland earthquake struck close to the northern end of the trench, releasing 25,000 times more energy than the atomic bomb dropped on Nagasaki in 1945 and twisted the South Island, moving Puysegur Pt on the southwestern tip of the island 30cm closer to Australia.
Its remote location, near Dusky Sound, meant there was relatively little damage.
A magnitude 7.2 quake hit the trench itself in November 2004.
"This earthquake was certainly associated with the subduction processes in some way, but the exact fault that it would have been located on is quite hard for us to tell right now," GNS seismologist Dr Anna Kaiser said.
"The Pacific Rim is obviously a really active margin for earthquakes and volcanoes, and this [subduction zone] is certainly part of that system."
Kaiser added there was nothing to suggest there was any direct link with the major quake that hit Mexico City.
"It's certainly something that can't be scientifically proven."
There was much more scientists could learn about the southern quake zone, but its far-flung location had made it a difficult area to investigate.
The Hikurangi Subduction Zone, off the North Island's East Coast and now the focus of a major international project, ultimately posed the greater threat to people.
