New Zealand scientists warned South Island residents to expect a swarm of aftershocks following a strong early morning six-magnitude earthquake Tuesday that caused no reported damage or injury.

The jolt struck at 6:48 am (1748 GMT Monday) in a sparsely populated area more than 100 kilometres (60 miles) west of the city of Christchurch, which was devastated by a 6.3-magnitude quake in 2011.

The latest temblor was initially measured at 6.4 by the New Zealand monitoring service GeoNet before being downgraded to 6.0. The US Geological Survey measured the tremor at 5.6 at a depth of 10 kilometres.

It was centred 30 kilometres (18 miles) from Arthur's Pass on the western side of the Southern Alps that run the length of the island, after initially being placed near Methven on the eastern side of mountain range.

"The intensity of this quake is considered severe," GeoNet said in a statement, adding early location and size recordings were skewed by a small foreshock five seconds prior to the main shake.

Within four hours, 30 aftershocks up to 4.2 had been recorded in the remote alpine region and "we expect aftershocks to continue", GeoNet said.

"In typical aftershock sequences, we can expect the largest aftershock to be up to magnitude 5.0."

Earthquake-prone New Zealand sits on the edge of the so-called Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.

The country experiences about 20,000 tremors a year, with an average of 2.5 of magnitude 6.0 or higher.

In 2011, 185 people were killed when a devastating 6.3-magnitude quake hit Christchurch in one of the nation's deadliest disasters of the modern era.

Wellington was the scene of the country's most powerful earthquake in 1855.

That 8.2-magnitude quake caused four deaths and changed the city's entire geography, pushing the shoreline out 200 metres (650 feet) as it thrust the harbour floor upwards.