The Commerce Commission strayed beyond its remit in its initial negative reaction to a proposed merger by Fairfax and NZME, the media companies say.
The commission last month said it was inclined not to approve the merger of the country's two major media chains because, among other things, it would lessen the diversity of news outlets.
The commission has received more than 30 additional submissions for and against the proposed merger, after it cast doubt on whether the public would enjoy the same level of diversity of news and information, if control of most of the country's newspapers, a leading radio network, and two major news websites came under one banner.
NZME and Fairfax said that regulating media on the grounds of diversity was outside the commission's jurisdiction, and it should focus on only the economic impacts.
They said the commission had misunderstood the level of competition that exists apart from its traditional competitors, such as TVNZ, MediaWorks and RNZ.
They said the level of news coverage on Facebook and Google was a game-changer for the industry.
That view has been backed by a large number of additional submissions from people or organisations, some of whom are associated with the media companies.
They've warned that if the merger doesn't go ahead, then they would be forced to significantly reduce their investment in front-line journalism, which would include regional and community reporting.
The commission will hold a three-day public conference next week to look at the submissions and the proposed merger.