Residents across the South Island have been rattled on Tuesday night, after an earthquake south of Stewart Island set the region shaking.

The earthquake confused GeoNet's systems, originally registering as a magnitude 6.1 near Invercargill, before being revised to a magnitude 5.0 near Blenheim, at the other end of the island, at around 7pm.

It's since been revised yet again to a magnitude 6.4 south of Stewart Island, at a depth of 34km.

The quake was most strongly felt in the south of the South Island, with more than 2000 people reporting feeling weak to moderate shaking.

People on Twitter have described feeling a long, rolling rumble in Invercargill, with city councillor Darren Ludlow telling Newshub it lasted around 20 seconds.

The Ministry of Civil Defence says there is no tsunami threat to New Zealand after the quake, however identified it as a 6.8 magnitude.

Newshub.