The teal was found on 28 December in an area out of bounds to the public and its body was sent away for tests which showed it had the H5N8 strain of the infection.
It follows cases in south Wales but Public Health Wales has previously said the public health risk was "very low".
The Welsh Government said the latest finding was "not unexpected".
A spokesman added: "It follows increased surveillance and calls for the public to notify any finding of dead geese, ducks and swans to the Defra helpline".
A wild duck has died from avian flu at an RSPB reserve in Conwy, it has been confirmed.