Milford Sound. Photo: 123RF
MetService says the waters around Fiordland may reach the highest temperature ever recorded.
An extreme marine heatwave has been affecting the lower South Island, with sea temperatures expected to peak in the coming week.
MetService oceanographer Dr Joao de Souza said Fiordland's may reach six degrees higher than normal, which would set a record for the region.
"The ocean has broken all records - the longest marine heatwave, which lasted just over a year in the Bay of Plenty, and now the biggest departure from normal, with 23-degree warm surface waters off Haast," he said.
"This is similar temperatures to waters off Northland, and normally never happens at these latitudes."
He described the findings as "disturbing" and said a six degree anomaly was "a whole new level compared to what we have seen before".
Dr Robert Smith, Moana Project oceanographer from the University of Otago, said the warm water was likely caused by a mixture of climate change and the ongoing La Nina conditions.
"A blocking high pressure system with light winds in the area is leading to reduced vertical ocean mixing and reduced heat loss from the ocean, triggering the marine heatwave," he said.
"This is exacerbated by warming in the eastern Tasman Sea, which is currently more than two degrees warmer than normal at depths of 100-400 metres.
"These warm underlying conditions makes it easier for the surface ocean to tip over into marine heatwave conditions."
Commercial fishing operator Craig Jones, who works with the Moana Project, said he had also noticed the effects of warmer water over the past few years.
"The patterns in fish are changing, the fish are still around but it's not the same," he said.
"This summer in particular we are seeing snapper and kingfish down our way more than we traditionally have. They seem to be trending south."
De Souza said that for the past couple of years, marine heatwaves had persistently impacted coastal waters, affecting sea life and the fishery industry.