The Welsh government has confirmed the presence of avian influenza H5N1 on a mixed poultry premise near Crickhowell, Powys.
The premises hold chickens, ducks, geese, turkeys, pheasants, swans and rheas, the Chief Veterinary Officer for Wales Christianne Glossop said on Sunday.
A 3km Protection Zone, a 10km Surveillance Zone and a 10km Restricted Zone have been declared around them to limit the risk of disease spread.
There have been dozens of similar findings of highly-pathogenic avian influenza in the UK in recent weeks.
On Monday, the Chief Veterinary Officers for England, Scotland, Wales and NI brought into force new mandatory housing measures to protect poultry and captive birds from bird flu.
Farmers and bird keepers are being strongly advised to be vigilant for signs of the disease such as increased mortality or respiratory distress.
If keepers have any concerns about the health of their birds, they should seek prompt advice from their veterinary surgeon.
Ms Glossop said: “This confirmation of a case of avian influenza in mixed poultry is further evidence of the need for all keepers of birds to ensure they have the very highest levels of biosecurity in place.
“New housing measures have come into force to protect poultry and kept birds, but I must stress that this is at its most effective when combined with implementation of the most stringent biosecurity measures."
She added: “This is the third case of avian influenza to be confirmed in Wales this winter so far, which highlights the importance for keepers of birds to be vigilant.
“Public Health Wales has said the risk to the health of the public from avian influenza is very low and the Food Standards Agency has made clear it does not pose a food safety risk for UK consumers."
Members of the public are being encouraged to not pick up or touch any sick or dead birds and instead contact the Defra helpline on 03459 33 55 77.