Residents reported a violent jolt that rumbled on for several seconds.
The quake shook Christchurch District Courthouse from side to side in Durham St in the city.
Jurors deliberating in the murder trial of George Gwaze asked court staff to be keep them informed as they did not have access to their phones or computers, but seemed unfazed and continued to deliberate without a break.
The central city red zone cordon has been evacuated, with up to 1000 rebuild workers and demolition contractors told to leave the danger zone.
Debris was reported falling from already quake-damaged buildings and more bricks reportedly came down off the crippled Christ Church Cathedral.
Cantabrians turned to social media to report their experiences on the latest shake - the 31st biggest since the earthquake sequence began in September 2010. Today's quake was the first jolt above magnitude 5.0 since January 15.
On TradeMe, one poster wrote: "Things fell off shelves and bench here in Linwood/city.''
Another said: "big here in Kaiapoi, freaked out the 2yr old.''
Christchurch business owner Rita Mason was having lunch at a restaurant in Merivale when the quake hit.
"It went on for quite a while, it kept rolling. It felt like a decent one.''
Other residents quickly took to Twitter, with Hamish Clark tweeting "Heart rate still going... waiting for any follow ups #eqnz''.
Christina Evans said : "Woah that's a wobbler! How y'all doing out there? #eqnz''.
" ... and that folks is a reminder we live in Christchurch #eqnz,'' said Nancy Kane.
The shake was followed by a 3.6-magnitude quake four minutes later, which hit at a depth of 20km, five kilometres north-east of the city.
