Two Canterbury lakes are closed as the Ministry for Primary Industries (MPI) looks into the cause of a rash found on a number of rainbow trout.
Fish and Game alerted MPI to the trout in Lake Grassmere, which has since been closed to anglers.
The nearby Lake Sarah has also been closed as a precaution.
An MPI spokesman said tests were being run on the fish to establish whether they were carrying an unknown disease.
The types of possible fish diseases in question do not carry a safety risk for people that eat the fish, he said.
The tests are expected to take around three weeks.