Christchurch has been rocked by another large earthquake, the biggest in magnitude since Monday morning's 5.5 shake.
The latest tremor was felt at 6.59pm on Monday and measured 4.8 on the richter scale.
It was centred 20 kilometres east of Christchurch at a depth of 15 kilometres, in a similar direction and at a similar depth to the quake which hit the city on Monday morning.
Police and the fire service say no damage has been reported.
Christchurch mayor Bob Parker is calling for an urgent briefing from GNS Science on the latest series of earthquakes to hit the area.
Mr Parker says it's important Cantabrians hear from the scientists themselves on what's happening and what is the likelihood of further events.
The morning aftershocks forced the closure of the Christchurch City Council offices and the high rise HSBC tower which is home to the Canterbury Earthquake Recovery Authority.
The buildings will remain closed until structural engineers have inspected them.
GeoNet has recorded 18 quakes between 1.27am and 7pm on Monday - the first measuring 5.1. The largest, measuring 5.5, happened at 5.45am and was actually a double shock 12 seconds apart.
Lines company Orion says the 5.5 tremor triggered a power shut-off to 10,000 homes in Christchurch's northeast, but electricity was restored by 8am.
All of the quakes have been relatively shallow at depths of between 8km and 20km, and centred east or north-east of the city.
Mr Parker says people can be assured that the Christchurch City Council is doing everything it can to ensure that key services remain operational.
Mr Parker told Radio New Zealand's Summer Report programme the latest quakes have been very unsettling.
The head of the Canterbury Earthquake Recovery Authority says there does not seem to be any significant further damage or liquefaction, but people are on edge.
"People just want a good night's sleep at the moment and when you're having a whole night of it, it's very, very difficult to get some sleep.
"People need to be looking after each other at the moment - going to neighbours and making sure that people are actually coping, because it's not a lot of fun."
In February last year, Christchurch was battered by a powerful 6.3-magnitude earthquake that killed 181 people and destroyed much of its city centre.
New Zealand sits on the so-called "Ring of Fire", the boundary of the Australian and Pacific tectonic plates, and experiences up to 15,000 tremors a year.
Anyone feeling distressed or overwhelmed by the aftershocks can call the Canterbury Support Line on 0800 777 846.