The Federal Court of Australia has decided that a person who published allegedly defamatory tweets on Twitter does not receive the benefit of the journalists' privilege under the Evidence Act.
See Kumova v Davison  FCA 753
This does not mean that a person who tweets can never be considered to be a journalist. In this case, looking at the Twitter feed as a whole, the defendant was not considered to be a journalist.
See this helpful note from Clayton Utz. Also Bennett & Co. Story in the AFR and The Age.