An earthquake measuring 6.3 in magnitude has hit New Zealand's South Island, hours after an initial quake killed two people.

The new tremor struck at 13:45 local time (00:45 GMT) at a depth of 10km (6 miles), northeast of Christchurch.

A 7.5-magnitude quake, with the same depth, had hit the same area just after midnight, triggering tsunami warnings.

A large river dammed up by a landslide has now breached its banks sending a "large wall of water" downstream.

Residents around the Clarence River – one of the largest on South Island – were being urged to move immediately to higher ground.

There have been hundreds of strong aftershocks, causing power shortages and cuts in water supply, since the initial quake.

GeoNet, a government-funded project monitoring earthquakes, said the first earthquake was actually two related tremors, and that aftershocks would continue over the next few months.