Authorities issue shark warning over whale that swam 11km upstream in Clarence River and later died

Juvenile humpback was spotted near Bundjalung national park in what experts say is a rare but not unprecedented event

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While the case of a whale found 11km upstream in a New South Wales river this weekend was unusual, it was not unprecedented, experts say.

The juvenile humpback, which swam into the Clarence River on the state's mid-north coast and later died, was towed out to sea on Sunday, according to NSW authorities.