Details of the issue have been captured in a November report from acting Environment Southland chief executive Rob Phillips, who referenced the recent state of the environment report.

Phillips said sea surface temperatures were increasing about 34 percent faster in New Zealand than the global average.

Outside of Fiordland, Southland was experiencing some of the fastest warming, with a 0.4 degree average increase over the past decade, he said.

“While it may mean that the region can enjoy some warmer swimming temperatures, continued increases will have detrimental effects on a wide range of our coastal ecosystems including valued fisheries and habitats.”

Council general manager science Karen Wilson provided further details, saying the ocean acted like a heat sponge, absorbing excess heat from the atmosphere and preventing dramatic rises in air and land temperatures.

Warming waters fueled extreme weather events and drove sea level rise, she said.

"Rising temperatures can also disrupt ocean chemistry and nutrient cycles, altering major currents and threatening marine ecosystems."

That could lead to coral bleaching, potential food security issues, and habitat changes for marine mammals, Wilson said.

Ocean temperatures fluctuated naturally due to El Niño and La Niña cycles, changes in ocean currents and long-term climate patterns, she said.

"These natural influences combine with broader climate changes and drive the warming we are observing today."

The 0.4 degree increase in average temperature also applied to the east coast of the South Island.

LDR is local body journalism co-funded by RNZ and NZ On Air.