Both of Monday's significant earthquakes in Canterbury were bigger than originally reported.
GNS Science has upgraded the quakes, saying the first at 1pm was magnitude 5.6, not 5.5.
It says the bigger quake at 2.20pm was 6.3, not magnitude 6.0.
Seismologists say the larger quake was as big as the earthquake on 22 February which killed 181 people.
GNS Science says the shaking caused by Monday's 6.3 magnitude aftershock was more than four times more powerful in Sumner than it was in central Christchurch during February's quake.
Seismologist Bill Fry says in February shaking was about half the strength of gravity, while in Sumner on Monday shaking was more than twice as strong as gravity - at 212%.
However, Dr Fry says part of the reason the measurements in Sumner are so high could be topographical effects on the ridge where the seismic recorder is based.