Mr Key, in Sydney for talks with business leaders, said whaling by South Korea would be a terrible step in the wrong direction and New Zealand would make its feelings known.
He said that although he understood South Korea was at this stage looking at the issue and would conduct a scoping study, there was a risk that if whaling began it could spread to international waters.
"We would hope that after this [scoping] analysis they would see sense and not move down that direction," Mr Key said.
"I think most people find the concept of killing whales abhorrent and I just can't see why anyone would want to condone that sort of activity."
Mr Key said the International Whaling Commission was already working with New Zealand and other countries to try to find a solution to Japanese whaling in the Southern Ocean.
"We were getting closer but unfortunately things are currently on hold," he said. "All I can say is that most people around the world would be very disappointed if the Koreans took this step."
My Key said he did not believe research whaling was necessary or appropriate, that stocks could not support such a move, and that it would not be good for South Korea's reputation.
"They have made the case they're doing it for scientific reasons.
"It's not for me to judge that, but historically we've been sceptical when Japan has used that as a rationale for whaling."
