Due to the shallow distance of the epicenter from populated areas, Tuesday's event is so far the largest earthquake to have occurred in New Zealand's Christchurch region in historic time, Australian experts said.

A massive 6.3 earthquake hit New Zealand South Island's largest city of Christchurch for the second time in less than six months.

The quake on Tuesday struck at a shallow depth of just four kilometers with an epicenter around 10 km outside Christchurch.

At least 65 people are confirmed dead in the quake, but the death doll is expected to climb as rescuers continue to look for survivors in buildings. New Zealand Prime Minister John Key said the nation may be witnessing "its darkest day".

According to Dr. Gideon Rosenbaum, a Lecturer in the School of Earth Sciences at Australia's University of Queensland, the distance of the epicenter from populated areas is the key to determine how destructive a magnitude 6.3 earthquake can be.

"As far as I understand, the earthquake today (Tuesday) was shallower and closer to Christchurch in comparison to the September earthquake," he said in a statement released by the Australian Science Media Center on Tuesday.

"The type of rocks in the affected areas are also very important. Hard rocks are stronger and more resistant, whereas soft rocks, particularly if wet, can amplify the seismic effect (a process called liquefaction).

"I think that Christchurch is built on silt that was affected by this process."

In July 2009, a magnitude 7.8 earthquake occurred in the remote southwest of South Island in New Zealand with lesser damage.

Dr. Gary Gibson, the Principal Research Fellow in the School of Earth Sciences at University of Melbourne said Tuesday's quake is by far the largest earthquake to have occurred in the Christchurch region in historic time.

He said in the last 200 years, it is more frequently to see large earthquakes occur in the plate boundary than in the big city of New Zealand, like Christchurch.

He added that the earthquake fault rupture in September began about 40 km west of Christchurch and ruptured for another 40 km heading off to the west, away from Christchurch.

"The earthquake this morning was smaller with a rupture of about 15 x 15 km at shallow depth immediately under Christchurch, so the shaking was much stronger," Dr. Gibson told the Australian Science Media Center on Tuesday.

Meanwhile, Australian Foreign Minister Kevin Rudd said a team of 30 highly trained personnel, including engineers, medics and dogs, are en route to Christchurch to help New Zealand begin search and rescue operations on Wednesday morning.