Christchurch City Council says there are new reports of liquefaction and flooding in the eastern suburbs following Saturday's 5.3 magnitude aftershock.
GNS Science reports the aftershock hit at 5.49pm at a depth of 11km and was 10km north-east of Diamond Harbour.
There was also a 4.1 aftershock, 5km from Diamond Harbour at 8.40pm.
There has been new liquefaction and flooding in the eastern suburbs, which was badly damaged in the earlier earthquakes.
Bexley resident Chris Holmes says he and his wife were out when the quake struck but came home to find their garage flooded.
He says the front of his driveway is covered in water which smells and liquefaction and water is all through their garage again.
The Strategy Building on Victoria Street, just north of the city centre, tilted during the aftershock, and extra cordons have been placed around it.
Montreal Street has been closed, and a diversion is in place.
There are also reports of new rock falls in the Sumner and Redcliffs area.
Rocks have fallen on Evans Pass Road.
Some power lines are down over a digger at the Stanmore shops at the corner of Stanmore Road and Worcester Street and the intersection is closed.
Christchurch mayor Bob Parker is once again urging people to conserve water after the aftershock broke some pipes and water mains.
He says a broken water main is feeding into the Redcliffs and Sumner area.
Mr Parker says Lyttelton residents also need to watch their water use and the reservoir, which has a low water level, is being checked.
Power back on
Immediately following the aftershock, about 20,000 customers lost power, but 99% had power restored by 8pm.
The lines company, Orion, says it has discovered a pocket of about 200 customers in south Christchurch who are still without power.
Chief executive Roger Sutton says Orion thought power had been restored to everybody but there was a problem with its computer system which did not register those still in the dark.
Mr Sutton says he hopes to have power restored to all customers by 11pm on Saturday and Orion will be looking into the computer problem on Sunday morning.
Christchurch Hospital says it lost power following the aftershock but its emergency generator kicked in.
The hospital says it was not affected by the power loss and has had no people through its emergency department as a result of the aftershock.
Police checked Lyttelton, Cass Bay and Port Hills after reports of minor rockfalls.
They say there is also some rock debris in the Marriner Street area of Sumner, although the road is not closed.
The Lyttelton tunnel remains open.
Police urge people to be careful on the roads and keep speeds down, particularly in areas where traffic lights or street lights are out.
Phone services were disrupted for a time and some mobile phone sites were forced to run off battery power.
Biggest aftershock since 22 February
The aftershock is the biggest earthquake since the 22 February quake hit Christchurch.
The biggest aftershock since the devastating 6.3 earthquake was on the same day of that quake, measuring 5.7.
There was also a 5.5 magnitude quake on that day.
The last aftershock with a similar magnitude was 5.1, on 20 March.