Utmost concern has been raised on the spread of the deadly Hendra virus in Australia that has so far killed a sixth horse since it occurred more than two weeks ago.

Reports confirmed that the a sixth horse succumbed to the illness, while 26 people who came in contact with the sick animals are now undergoing tests.

In the latest case, a horse fell ill and died at a farm south of Brisbane on Monday, not far from where other Hendra cases have been confirmed.

Six animals are now dead since the outbreak first emerged on June 20 - five in southern Queensland state and one in northern New South Wales related reports from Agence France Press said.

The rare virus can spread to humans, leading to fatal respiratory illness, and has killed four of the seven people who have contracted it in Australia since it was first documented in 1994.

"The recent spate of diagnosis of Hendra virus are deeply concerning,'"Queensland Premier Anna Bligh told reporters. "My heart goes out to the people who are waiting for these results."

In Queensland, 17 people who were exposed to the sick horses are waiting to learn if they have contracted the deadly virus. Nine others in New South Wales are also awaiting test results.