The first quake - magnitude 5.8 quake, 8km deep and centred 20km north east of Lyttelton - struck at 1.58pm (NZ time). The second earthquake, which residents said felt even stronger, hit at 3.18pm (NZ time)
New Zealand's second largest city was hit by a 6.3-magnitude quake in February, killing 181 people and destroying much of the downtown area.
A partly demolished building and a vacant house have collapsed following today's quakes, police said.
There were still no reports of widespread damage or injury, but there had been significant rockfall at Redcliffs, police spokesman Stephen Hill said.
A stopbank had also collapsed, and there were reports of major holes in several roads and liquefaction.
Earlier today, Hill said four people had to be rescued after they were trapped by rockfall in Boulder Bay.
One person, who was inside a shopping mall, had been injured and was taken to hospital.
Phone services were disrupted and about 15,000 customers were without power in the eastern suburbs.