My guess would be that some ISPs have one handover in Christchurch.
Spark and larger ISPs may have handovers in multiple exchanges, or some way of having diversity of aggregation with the UFB network.
2degrees initially went down, but they definitely have multiple handovers, perhaps something went wrong with the automatic failover or it required a manual change to move traffic away from the failure.
Complete stab in the dark without really knowing much about the UFB architecture or any inside ISP knowledge..