The tremor hit at 2:43 pm (0143 GMT) at a depth of 10 kilometres (6.2 miles) off the southwest tip of New Zealand's South Island, according to the USGS.
Centred around 160 kilometres (100 miles) from mainland settlement Riverton, it was initially recorded as a 7.0-magnitude jolt before it was downgraded.
There was no tsunami warning or immediate reports of damage, according to the Honolulu-based Pacific Tsunami Warning Center.
New Zealand's emergency management agency warned residents to avoid nearby coastal areas due to the risk of "strong and unusual currents".
"No land threat is expected," the agency said in a national advisory.
Rose Ivory, owner of La Riviera Guesthouse in Riverton, said her "whole car was rocking and rolling".
"That was quite a long one," she told AFP.
"Everyone was quite calm about the whole thing. I went into the grocery shopping, yeah, no one seemed to be too bothered about it."
Ben Sievwright, manager of Ziff's Cafe and Bar in Invercargill on the South Island, said he felt "a little sway".
"Just a little wee shake, nothing too eventful," he told AFP.
New Zealand straddles the boundary of two major tectonic plates and is rattled by thousands of small earthquakes every year.
A magnitude 6.3 earthquake levelled swathes of Christchurch in 2011, killing 185 people, according to government figures.
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