Whale Oil blogger Cameron Slater was acting as a journalist when writing allegedly defamatory posts about an Auckland businessman but must reveal his sources for the articles, a High Court judge has ruled.

The judge reached the conclusion that Slater was a journalist even though the Whale Oil publisher denied he was one in 2012, the year when the articles in question were released.

Slater is being sued for defamation by discharged bankrupt Matthew Blomfield, who did marketing work for Hells Pizza until 2008 and a was a director of a company that owned several of the pizza chain's stores.

Slater during five weeks in 2012 wrote 13 articles referring to Blomfield, who claims the Whale Oil publisher defamed him.

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The bulk of these articles contain extracts from emails allegedly associated with Blomfield and refer to files which the businessman says came from his hard-drive and potentially a file cabinet belonging to him.

Slater admitted publishing the articles at issue but denied they conveyed the alleged defamatory meanings and has also raised the defence of truth and honest opinion.

Blomfield in August 2013 applied to the Manukau District Court wanting orders that Slater disclose who supplied the hard drive and other information.

Slater opposed this on the basis he was a journalist and such disclosure would require him to reveal the "informants" whose identities he had promised he would keep secret.

Judge Blackie last September said Whale Oil was a blog site and the relevant sections of the Evidence Act - which give journalists certain protections from revealing sources - did not provide a basis for Slater to refuse disclosure.

Slater appealed to the High Court and in a decision released today, Justice Raynor Asher said the definition of a journalist under the Evidence Act could include a blogger.

"The Whale Oil website was a news medium in that it was disseminating new and recent stories of public interest. While its style and focus can be criticised, it was breaking news to a significant section of the New Zealand public," Justice Asher said.

The judge said that Slater was a journalist and could invoke source protection under the Evidence Act.

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However, the judge also granted orders sought by Blomfield that this section of the law not apply to Slater in this case.

There was a "public interest" in the disclosure of the identity of Slater's informants, Justice Asher said.

"There is a real public interest in those who claim that they are defamed being able to fully explore the circumstances of the defamation..." the judge said.

This was not a whistle blower case and it seemed the information was obtained illegally by sources, which diminished the importance of protecting them, the judge said.

"Moreover, any concern at the chilling effect of disclosure of sources is lessened when the subject matter of the material originally disclosed has the mark of a private feud, and features abusive and vindictive language."