The commission's final decision, due this morning, comes almost a year after the two companies applied for authorisation and would still be subject to shareholder approval.
The commission, in a draft decision last November, indicated its initial view was that it would reject the merger.
The regulator, at the time, said its initial view was the media companies' merger would be likely to substantially lessen competition in a number of markets, including the markets for premium digital advertising, and advertising in Sunday newspapers and in community newspapers.
The commission also said that the check that NZME and Fairfax provide on each other would be lost in the merger, as would different perspectives and voices.
After the draft decision, the commission hosted a conference where Fairfax chief executive Greg Hywood argued that if the merger is disallowed, it would "become end game" for Fairfax NZ's media assets.
During the conference NZME and Fairfax argued they were not each other's major competition, and other organisations including TVNZ, Radio NZ and MediaWorks were strong competitors in a "digital first" environment where traditional print and broadcast boundaries were broken down.
Facebook was also taking a growing share of advertising, despite not producing news content.
Shayne Currie, managing editor of NZME, said Facebook founder Mark Zuckerberg was "by far the world's most powerful editor" and presided over a "phenomenal beast" that was "100 per cent our main competitor and constraint".
TVNZ and MediaWorks have opposed the merger and during the conference said they did not consider themselves to be able to be a competitive constraint on the proposed merged company any time soon.
Others at the conference also spoke out against deal and former Herald editor Gavin Ellis said there was a danger that a merged entity would lead to decisions being made by fewer people, but affecting more publications.
NZME and Fairfax, in their final submissions to the commission in February, said differences between those who are for and against the merger was "a contest betwen hard data on the one hand, and wishful thinking on the other".
The lawyers for both companies said they had submitted extensive data about share of the advertising market, market trends in revenue, synergies, and efficiencies, and accounting expert reports on the logical and necessary conclusions from the data.
"In contrast, a large number of submitters in opposition have presented the commission with their views as to why the businesses have not been sucessful (although they have not run those businesses in the recent - accelerating - market conditions).
"They have also expressed hope that different combinations of businesses in market might present a better alternative to the proposed merger."
But the lawyers said the proposed merger had not been put to the commission lightly or early.
"The necessary corollary of that is that it cannot be put to the commission later, because later will be too late."
Any party who spoke during the conference can appeal the commission's final decision.
