It looks like the quoted epicentre was just the start point of the rupture that headed off to the northeast. Which explains why the shaking seemed worse to the north and why so much activity seems to be recorded from the Seddon area again:
http://earthquake.usgs.gov/earthquakes/eventpage/us1000778i#...
In Wellington it seemed like the first half was just a gentle swaying before it really kicked in for the second half. That was probably the rupture moving closer.
I found this really interesting: The USGS talks about quakes of this magnitude usually have a wide scale area of slippage eg they've modelled 200x60 km for this one:
http://earthquake.usgs.gov/earthquakes/eventpage/us1000778i#... http://earthquake.usgs.gov/earthquakes/eventpage/us1000778i#...