Please, the animals are adaptable. It doesn't turn deathly cold in minutes. They have nests and places to bed down. In the case of fish cold water doesn't overwhelm ambient temperature in seconds.
How does cold explain the what's going on in New Zealand?
As I noted, most of the kills are in the Northern Hemisphere. Other factors can cause kills, such as pollution, red tides, etc. But the cold can explain most of the kills in the Northern Hemisphere.
Uh, yes, it can get cold. And birds only have nests to raise young, not as shelters.
In the case of fish cold water doesn't overwhelm ambient temperature in seconds.
But a severe cold snap of several days can lower temps water quickly and shock their systems. Fish are used to gradual changes.
Perhaps volcanic and tsunamis are what is going on in the Southern hemisphere.