The highly contagious and deadly H7N8 strain of bird flu has been detected at a second farm in the Hawkesbury region, NSW's health minister says.

More than one million birds will be destroyed across seven properties in Victoria to tackle the continuing spread of bird flu.

Australia's largest outbreak of bird flu has hit a grim milestone in the spread of the virus that was first detected in May.

Hundreds of thousands of birds have been killed on a poultry egg farm after authorities detected bird flu on the property.

A commercial farm in NSW is under lockdown after bird flu was found in chickens, although the strain is different to the outbreak impacting Victoria.

A commercial farm in NSW is under lockdown after bird flu was found in chickens, although the strain is different to the outbreak impacting Victoria.

A commercial farm in NSW is under lockdown after bird flu was found in chickens, although the strain is different to the outbreak impacting Victoria.

A second chicken farm at Hawkesbury on the outskirts of Sydney has been infected with bird flu, prompting a mass cull.

The major supermarket will be limiting the number of egg cartons customers can buy as the industry continues to be hit by a deadly strain of bird flu.

Authorities are urgently confirming whether a large-scale bird flu outbreak in one state has spread across the border.