Two strong earthquakes rattled the South Island overnight.

The first, at 11.32pm, was a magnitude 4.5 earthquake which struck just north of Tai Tapu, 10km southwest of Christchurch, at a depth of 11km.

GNS Science had measured the quake as a magnitude 5, but later revised it down.

The quake was felt strongly around Christchurch, and was reported as "slightly damaging" in Fendalton and Hoon Hay.

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Just a minute later, a quake of a similar size struck near Blenheim.

The tremor, a magnitude 4.6 earthquake, was centred 10km south of Havelock and 20km southwest of Blenheim, at a depth of 70km.

GNS Science received reports it was felt across the Marlborough, Nelson and Wellington regions.

- Herald Online