Society of Authors president Kyle Mewburn said the new ruling was "unsettling" and should be reversed by Parliament.
"There have been a lot of exposs over the years. If they are stymied by legal concerns and people not willing to talk to them because of risks of being identified, then that is a fundamental issue of democracy," he said. "The act needs to be readdressed if that is the case."
Christchurch Press editor Joanna Norris, who chairs the Commonwealth Press Union's NZ media freedom committee, said Justice Winkelmann's judgment was "an arguable interpretation".
"Consideration now needs to be given as to whether the law needs to be amended to take into account the spirit of the legislation, which is intended to allow journalists to protect their sources and source material," she said.
Justice Winkelmann's ruling relates to a civil claim by Mr Dotcom, his estranged wife Mona and others for $6 million in damages from the police and the Government Communications Security Bureau for a raid on his Coatesville mansion in 2012 and alleged illegal surveillance.
The judge upheld an argument by the police and GCSB that information held by Fisher should be provided to them under legal discovery rules because the Privacy Act gave Mr Dotcom a right to access all information that Fisher held about him.
Canterbury University law professor Ursula Cheer, the co-author of News Media Law in NZ, said the ruling was "an unusual interpretation" which came down to the Privacy Act's definition of "news activity" as the gathering, compilation and dissemination of "any article or programme" of news.
"The word 'book' is missing," she said. "You could say that that needs to be looked at again."
However, she said the ruling did not affect the main provision allowing journalists to protect their sources from being disclosed in legal proceedings, which is in the Evidence Act.
That act does not define "news activity" but defines a journalist as someone working for a "news medium", which is defined as "a medium for the dissemination to the public or a section of the public of news and observations on news".