Tens of thousands of hens will be culled, threatening the supply of eggs in shops as a deadly bird flu strain spreads to a third Victorian poultry farm.

Avian influenza strain H7N3 has been detected at a Lethbridge farm owned by Farm Pride in the state’s south-west, sparking a safety alert for all bird owners including farmers and households with back yard chickens.

The positive test was confirmed on Monday and about 80,000 hens, representing 8% of the company’s total production capacity, will be euthanised to prevent the spread of the virus.

The managing director of Farm Pride, Darren Lurie, said the company continued to operate strict biosecurity measures and has been following the agreed national response against emergency animal diseases.

“Farm Pride places paramount importance on the quality of our product, food safety, staff welfare, protecting our customers, suppliers and stakeholders, and the health of our hens,” Lurie said in a statement.

Danyel Cucinotta, the vice-president of the Victorian Farmers Federation and an egg farmer, said the industry was doing everything possible to keep up the supply of fresh and affordable eggs.

“We’re anticipating a flow-on impact to egg supplies in the coming week and are working as hard as possible to maintain availability,” Cucinotta said.

“My advice is to shop around at your local grocer, market or small independent store to buy your eggs.”

An alert issued by Agriculture Victoria states consumers should not worry about supermarket eggs and poultry products because they do not pose a risk and are safe to consume.

The latest farm caught in the outbreak is within an existing five kilometre exclusion zone set up after the virus was detected at a nearby property in May.

About 500,000 birds were culled after avian influenza was detected at two farms operated by Avgo and Surf Coast Eggs Farms, which share management, staff and machinery across sites in Meredith and Terang.

Victoria’s chief veterinary officer, Graeme Cooke, said the latest discovery was “not unexpected”.

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He urged all bird owners in the restricted area to follow strict rules and immediately report any unexplained bird deaths.

“Regardless of whether you are a commercial producer, or you keep a few chickens in your back yard, there are things you must do to protect your birds from disease,” Cooke said.

There’s more than one bird flu: what recent outbreaks mean for Australia – video

The restricted zone in Meredith has been expanded and a buffer zone is in place at the eastern part of the Golden Plains shire council, meaning a permit is needed to move birds, poultry products, food or equipment around the area.

Cucinotta said poultry farmers were focused on identification and containment measures to safeguard their animals.

“Farmers have the ability to assess their own farm’s risk via their biosecurity plan and house poultry indoors for the coming weeks should they deem necessary,” she said.

The US has placed restrictions on imports of Victorian poultry and poultry by-products until further notice.