There were no reports of serious damage from the quake which hit at 9.26 am (2026 GMT Monday) about 240 kilometres (145 miles) off the southwest coast, at a depth of 33 kilometres.

The quake was felt widely through the South Island, and as far north as New Plymouth in the North Island which was on the same fault line.

"This was a large earthquake in global terms," seismologist Warwick Smith said.

"Had it occurred under or near a population centre there would be major damage."

But its depth and distance offshore meant that by the time seismic energy had reached land it was relatively weak and damage appeared to be confined to goods falling off shop shelves.

However, GNS spokesman Geoff Clitheroe, said there will "definitely be aftershocks".

Of the thousands of aftershocks expected over the next few weeks, only those over a magnitude of 5.0 or more -- probably numbering 10 to 20 -- would be felt.

One tremor of 3.8 on the Richter scale was recorded in the same area about two hours after the first.

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