A strong earthquake that hit Christchurch overnight has had its magnitude upgraded by GNS Science.

Initially it was believed the tremor, which hit just after 11.30pm, measured 4.4 in magnitude, but that has increased to 4.6.

It was also shallower than first believed, with GNS reporting it was 8km deep rather than 11km.

The quake was centred 10km south of the city.

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