Poultry farms in Western Australia have gone into lockdown after confirmation the deadly H5N1 bird flu has arrived on the country’s mainland.

On Monday, the Ingham’s Group – Australia’s largest poultry producer – announced a “complete lockdown” in WA, despite no commercial detections of H5N1.

It came after a brown skua, found on a remote beach near Esperance, was discovered to have the “highly pathenogenic” H5N1 virus on the weekend. A giant petrel in the same area was also tested and returned a preliminary positive result, while there have been reports of more than a dozen cases of sick or dead birds along the WA coast.

Before the confirmation of H5N1’s arrival, Australia had been the only continent free of the virus, which has killed millions of birds and thousands of marine mammals since 2021.

Ingham’s said in a statement it would ask the state government to allow free-range chickens to be kept indoors, while all nonessential access to its operations would be stopped.

Australia’s chief veterinary officer, Beth Cookson, told ABC’s Radio National the virus had not been detected in wildlife, poultry or agricultural systems. But Cookson said authorities were trying to work out whether the infected birds had spread the disease into other populations.

“Our approach is to really learn from the overseas experience and look at the practical actions that can be put in place to mitigate the impacts as far as possible,” she said on Monday.

Shares in Ingham’s dropped as much as 14% on Monday, according to Bloomberg.

The share price has been sliding for four months and was down more than 23% in the year to date, according to the news agency. The Australian Financial Review reported on 1 June that the $777m company was “deep in turnaround mode”, after a contract with Woolworths was restructured.

The federal environment minister, Murray Watt, said Cookson and the threatened species commissioner, Dr Fiona Fraser, would brief state and federal environment ministers about the virus on Monday.

“We’ve been working very hard with states, industry, environment groups [and] scientists over the last couple of years to make sure that we are as well prepared as we possibly could be,” he said.

“As a government, we’ve invested $113m, including $11m in the most recent budget in those preparedness efforts. So I feel confident that we’ve got the systems in place, and that we’re working cooperatively with states, territories and others to make sure that we can manage this outbreak if it does get more serious.”

– with Australian Associated Press