Severe aftershocks have struck near Seddon, causing strong shaking.
GeoNet initially recorded the first quake as two separate events, but it has since been revised to one tremor.
The 5.7 shake hit near Seddon, at a depth of 20km, at 2.34pm.
A second aftershock of 5.2 struck near Seddon again at a depth of 12km at 2.43pm.
Rail services have been suspended until further notice following the earthquakes.