The water in the area is about 5 degrees Celsius (9 degrees Fahrenheit) “warmer than average for the latitude and time of year,” said James Renwick, a professor and Head of the School of Geography, Environment and Earth Sciences at Victoria University in Wellington, New Zealand.

The patch of sea is about a million sq kms (400,000 sq miles), covering an area larger than the size of Texas.  “The ocean surface doesn’t vary that wildly,” Renwick said. “One degree is big. So, five degrees is huge.”

https://www.cnn.com/2019/12/27/world/new-zealand-hot-ocean-water-trnd/index.html