A 4.2 magnitude aftershock has struck near Christchurch this morning, six months on from the devastating February 22 earthquake.

The quake struck at 8.38am, and was centred 10km southeast of Lyttelton, in Port Levy on Banks Peninsula, at a depth of 10km.

GNS Science said the quake was "widely felt in Christchurch", with reports it was felt strongly in Bexley, Burwood and Redcliffs.

This follows an active weekend of aftershocks, including a magnitude 4.2 at 5.14am on Saturday and a magnitude 4.0 at 7.31pm on Saturday.

Questions over how CTV building collapsed

Meanwhile six months on from the devastating quake, questions remain over how the Canterbury Television building collapsed.

Out of the 181 who died in the disaster, 115 were in the ill-fated CTV building, which pancaked following the 6.3 magnitude earthquake, with only the lift shaft left standing - a haunting symbol of the tragedy.

The collapse of the building will be investigated by the Royal Commission of Inquiry, but structural engineer Jose Restrepo, from the University of California in San Diego, said the building's problems go right back to its construction.

Restrepo was shown the building's 1986 plans by TVNZ's Sunday, telling the current affairs show there were "critical areas" which would have contributed to the building's demise.

He described the way the floors were connected to the lift shaft - in some places using "brittle" wielded steel mesh - as "precarious".

"It just fractures - it has no ductility," he told Sunday. "It just takes a little pull and it snaps."

The former University of Canterbury professor now conducts earthquake engineering tests in the United States, and said the mesh would have been ripped off the shaft - which he said provides lateral stability for the building - and it came down.

Restrepo said the council appeared to have had concerns at the time of the building's construction, writing to the design engineers for more information about how the building would be connected to the lift shaft.

He understood there had been no written response to those concerns, although the response may have been verbal.

Restrepo also had concerns with the building's columns which he said could have "exploded in compression, rendering the building useless".

He believed there was not enough steel in the columns to hold them together, and the lower columns exploded after the building lifted during the quake.

"I can see from the photos, the building has not spilled to a parking lot or to the street. It has basically imploded."

Restrepo said the mesh may have been compromised in the September 4 and Boxing Day quakes.

The building was green-stickered after both quakes, but Christchurch City Mayor said it was the building's owner to ensure the building was safe after the shocks.

Restrepo stressed the building met building standards when it was constructed, but believed it would not meet today's standards.

"It's a terrifying story for that building."

The building's structural engineers Alan Reay Consultants principal Doctor Alan Reay was not prepared to speak to Sunday on camera, but said they are working with the Royal Commission.