A strong earthquake has struck south of New Zealand, between Snares Island and the Auckland Islands.

Geonet said the quake was a magnitude 6.4 that struck 190km south-west of Snares Island at 7pm.

Ok that South Island shake-up was a M6.4, 190km South-West of Snares Island (below the South Island) #eqnz #sorryfortheconfusion

— GeoNet (@geonet) July 11, 2017

The quake had "confused" GeoNet's instruments, with the service initially reporting a 5.9 magnitude earthquake had struck near Invercargill and another near Blenheim with a magnitude of 5.0.

Looks like a large quake below the South Island has confused our system a bit, our duty officer will have it tidied up ASAP #eqnz

— GeoNet (@geonet) July 11, 2017

There was no tsunami threat from the quake, Civil Defence said.

There is no tsunami threat to New Zealand following the 6.8 magnitude earthquake in the Auckland Islands. #eqnz

— MCDEM (@NZcivildefence) July 11, 2017

Although the true quake was offshore, it was felt by people around the South Island.

Residents from the South Island have shared their experiences on social media. Many reported swaying or shaking that lasted for more than 30 seconds and set light bulbs swinging.<

That was quite a big rolling one in Invercargill just now #eqnz

— Michael (@mjwkiwi) July 11, 2017

I felt that shake too. From Dunedin #eqnz

— Redzombie (@RedZombie20) July 11, 2017

Fire and Emergency New Zealand had received no reports of structural damage, a spokesman said, but the quake was felt at the fire station in Invercargill.

One person reported their car had sunk into liquefaction at their property in the Invercargill suburb of Glengarry but it appeared to have been "more or less a false alarm", he said.

"The fire crew have described it as one car stuck in the mud. So as far as we can tell it had very little if anything to do with the earthquake."

Neither the Snares Island and the Auckland Islands have permanent inhabitants, although scientists sometimes visit for research purposes.

The Snares Islands are the northernmost of the country's subantarctic islands and one of the "most untouched and pristine areas in New Zealand", according to the Department of Conservation, because introduced land mammals were never established there.

They lie around 100km south-west of Stewart Island.

The Auckland Islands lie 465km south of Bluff, and are the country's largest subantarctic islands. They are made up of the remains of two ancient volcanoes.

The islands are now predominantly conversation land with a marine mammal sanctuary established in 1993 and a marine reserve classification in 2003.