Freshwater anglers are being asked to keep an eye out for pest species after a grass carp was caught in Lake Dunstan.

It is believed the carp was released into contained ponds near Queenstown, but may have escaped during spring floods and swum down river into the lake, Otago Fish and Game say.

Central Otago operations manager David Priest says it is important for any pest fish species to be reported to Fish and Game to protect valuable fisheries.

"We appreciate the angler notifying us of this unexpected catch. Any further sightings of grass carp in Otago waterways should be reported.

"Thousands of anglers have been enjoying fishing for trout these summer holidays and harvesting wild kai from our Southern Lakes fisheries.

"The last thing we want is a disease entering highly valued fisheries."

Grass carp can not readily breed in New Zealand outside of captivity so there is no real risk of establishing a self-sustaining population in the wild, Mr Priest says.

They are a herbivorous fish native to Asia, that can weigh 10 kilograms and commonly grow more than half a metre in length.

Grass carp are artificially bred and grown on fish farms licensed by the Ministry for Primary Industries and have been used in parts of the country to control aquatic weed growth.

But a permit is required to move grass carp and they have to be contained in the water body where they are released.

Mr Priest says one of the major concerns over the use of grass carp in New Zealand is their potential effect on trout fisheries in lakes.

"Grass carp released into contained waters had the potential to host infection and disease which could then be carried and transferred to other waters or species."

The fish species have been released in a wide range of locations in the North Island but their distribution is more limited in the South.

MPI most recently authorised the release of 100 grass carp to artificial ornamental ponds near Queenstown in February last year.

They have been used for aquatic weed control in a select number of contained ponds in the area since 2013.

Main image (Supplied/Otago Fish and Game): A grass carp caught by an angler Louie Macandrew in Lake Dunstan over the holiday period.