UPDATE: 4PM
Six people have been arrested for looting in Christchurch, as police set up a curfew in the devastated central city.
Superintendent Dave Cliff said there would be a 6.30pm curfew inside the central business district, allowing access only to those involved in the rescue effort, NZPA reported.
"That's also about keeping out the criminal element, who we know will try and take advantage," he said.
"We have made about six arrests today for theft and burglary around the central city."
Australian police officers will be on hand to help stop looters.
"They will be there to provide support in general law and order, to ensure that general safety is being attended to and also to assist in tragedies like this," NSW Police Commissioner Andrew Scipione said.
"Unfortunately we see the best of human nature but we also see the worst.
"We see potential looters; we need to make sure that we are helping our colleagues in New Zealand to prevent that as often as we possibly can."
Christchurch's terrifying misery continues today as aftershocks continue to shake the city - sometimes as much as every few minutes - with victims still trapped and rescuers racing against time to find them.
The city and surroundings have endured around 5000 aftershocks since the September earthquake.
Between 7.30pm and 10am today, another 30 aftershocks were felt, many close to the city, Lyttelton and some near Diamond Harbour and Akaroa, according to data collected by GeoNet, which records earthquakes in New Zealand.
The shocks ranged from magnitude 3.2 to magnitude five.
Professor Malcolm Sambridge, Australian National University's head of seismology and mathematical geophysics, said the shocks might have felt like trains speeding by or sounded like claps of thunder.
More aftershocks are expected to rock the area for some time, Professor Sambridge said.
"We do expect earthquakes to continue for weeks and months as a basis of this, but it's not possible to determine whether this is a pre-shock for a bigger earthquake, but it's unlikely I think," he said.
While yesterday's 6.3 magnitude earthquake was considered an aftershock of the September 4 quake, which measured 7.1 in magnitude, it created smaller aftershocks of its own, along with continuing shocks from last year, he said.
Aftershocks are caused when earthquakes disturb the entire stress field of the earth's crust, which can put pressure on other minor faults.
"They're all earthquakes, an aftershock simply means ... the reason it occurred is related to some previous event," he said.
"They're all aftershocks of the main event but this event itself would overprint that and create other aftershocks because of its size."
Another big earthquake is possible, he said.
"It could happen again, but it wouldn't necessarily be an aftershock of this one. All things are possible.
"This earthquake has released energy and it's likely to produce several [magnitude] fives, many more fours and threes and so on.
"It is an unstable region, what can happen is when one earthquake occurs, it can pre-stress other parts of the fault, and related faults."
He said the timing of earthquakes can be hard to predict, but magnitude and frequency can be determined by statistics.
"Often, most parts of the earth's crust are close to failure.
"So when one earthquake occurs, it tips other areas and pre-stresses other parts of the crust and they can occur later.
"But predicting them in any real sense is impossible."
Several people took to Twitter to describe the aftershocks.
"Most people in Chch are shattered. No sleep cos of aftershocks & people worried for loved ones," said New Zealand's Opposition Leader, Phil Goff (www.twitter.com/phil–goff).
Another man (http://twitter.com/Altera-Nova) tweeted: "Had a crappy night. Some of the aftershocks rocked me wrong."
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