Bird flu has spread across state borders to South Australia, marking an alarming development since the deadly H5 strain was first detected in Western Australia last week.

Testing is still under way on a second bird after Australia's first case of the deadly H5 bird flu was confirmed on WA's south coast.

The H5N1 variant of the virus was confirmed in a sick brown skua found on a remote beach near Esperance in Western Australia, about 700km southeast of Perth.