A marine heatwave could result in different species of fish turning up in unusual places.
Snapper - typically a fish found in northern waters - are already being found down south.
And in the north, more tropical fish may be found.
Keen fishermen could find themselves pulling up unexpected species this summer.
"There's reports of fishermen catching snapper in Fiordland and kingfish further south," said NIWA programme leader of fishery and ecosystems Darren Parsons.
That could be down to several factors including overfished stocks rebuilding.
But their southern movement may also link to marine heatwaves - a period of abnormal marine temperatures for more than five days.
"We're just starting to see sea surface temperatures rise in a number of locations around New Zealand to the extent that we're already in marine heatwave in some parts of the country," said Victoria University professor of marine biology Dr James Bell.
The north of New Zealand will likely experience the warmest temperatures.
"You may see some new species appear there, tropical vagrants we refer to them as," Parsons said.
And as the cooler southern waters start to heat up, other fish will swim down.
"You may see a range of extensions occurring so john dory or snapper may start to creep further south," Parsons said.
Relief for roaming fish, not so much for other ecosystems.
"Mass sponge bleaching and mortality and necrosis of sponges up in Northland and also Fiordland as well," Dr Bell said.
Scientists are looking to the future and trying to plan for what's to come.
"Our forecast for the mid-century our temps are going to be 1.5C warmer than they are and the end of the century 2C warmer than usual," Moana Project science lead Jaoa De Souza said.
Climate change - bringing rising seas and sea temperatures.
