University of Otago Portobello Marine Laboratory data shows the temperature of the water in Otago Harbour has been sitting above 20 degC since last Thursday, and spiked to 21.98 degC on Sunday.
It has since dropped back to about 20 degC.
The warm harbour temperatures are being caused by extreme marine heatwaves in the ocean off the southern South Island described as "literally off the scale".
MetService oceanographer Dr Joao de Souza said the present heatwave was concentrated off Otago and around Stewart Island, bringing temperatures up to 6 degC warmer than normal.
"Extreme marine heatwaves are just that — extreme. It’s the highest category in existence for marine heatwaves, reflecting how unusual this level of warming is, both in New Zealand and internationally."
