A significant aftershock in Christchurch cut power supplies to several suburbs for a short time on Wednesday morning.
The 4.1-magnitude tremor struck at 6am on Wednesday, at a depth of 7 kilometres.
GNS Science reports it was 10 km south-east of Christchurch and within 5 kilometres of Lyttelton.
Power was out in the Heathcote Valley, Woolston, and parts of the eastern hill suburbs for about 20 minutes after the quake tripped a substation.
GNS Science seismologist Bill Fry says the quake had quite a large impact because it was so shallow and because the valley has already been so badly affected by the quakes.
"The area around Heathcote Valley ... has experienced a lot of shaking, a lot of side amplification, so even a distant earthquake has been felt more strongly than other places."
Dr Fry says Wednesday's tremor is a normal part of the process of aftershocks.