A mysterious health condition could be plaguing a pod of killer whales off the Queensland coast, with authorities finding the third dead whale in under a week.

The latest casualty became stranded off Fraser Island late last week, after two other orcas died on Wednesday when their pod became stranded on a sandbar in the Great Sandy Strait.

Emergency workers fought to re-float ad save 5 other whales stuck on the beach over the weekend.

Sea World Marine Sciences director Trevor Long said they are working on a possible cause; “Whether we're looking at a health situation with this group, it's hard to say at this point in time," he said, “unfortunately we haven't been able to get a freshly-dead animal to get anything really valuable from the necropsy... we're assuming that it may actually have been dead before it stranded.”