The investigation, under Section 47 of the Commerce Act, would examine whether the new pay TV platform Igloo, a deal between TVNZ and Sky Network Television, breached merger rules.
"Despite previously being a strong voice advocating more competition in the broadcasting sector and calling for a converged regulatory approach, TVNZ has in recent months gone curiously quiet on these issues.
"The silence follows its announced deal with Sky late last year which gives it a foothold in the pay TV market," Ms Curran said.
Igloo aims to launch a cut-price pay-TV service, providing 11 channels for about $25 a month.
A commission spokeswoman said Section 47 related to commission approval for a merger.
"A person must not acquire assets of a business or shares if the acquisition would have, or would be likely to have, the effect of substantially lessening competition in a market."
She declined to define the issues faced by Igloo in the investigation.
The commission was aware of timing for the planned start-up of Igloo in May or June, and the investigation would take account of those plans.
She declined to specify which market was being examined and whether it included dominance of the television content market, saying this would not be spelt out in an investigation.
The Igloo announcement in December marked a growing alliance between Sky and TVNZ.
The New Zealand Herald reported that while ostensibly a joint venture, Igloo was largely controlled by Sky TV.
It gave the state broadcaster a valuable foothold in pay TV, including the advent of internet downloads.
For Sky it created a "Sky Lite" package for people who did not want to make a commitment to a full package, but allowed them to be targeted to upgrade to a full service.
If successful, Igloo would likely make it more difficult for a rival pay TV venture to start here, which would be good for both Sky and the state broadcaster.
Ms Curran said given the convergence of the internet and broadcasting environments, that positioned TVNZ and Sky to potentially dominate the market in coming years and could prevent other existing and new players providing competition.
"Particularly relevant is the market power that Sky already has, as it also owns Prime, a free-to-air channel, something many countries do not allow to happen."
Sky TV and TVNZ both confirmed approaches from the Commerce Commission and said they were helping the regulator with its inquiries.
But Sky TV chief executive John Fellet said the Commerce Commission action would not affect the planned launch of Igloo in second quarter of 2012.
"It might affect the merger, but it would not affect Igloo. We were always going to do it," he said.
Ms Curran said there had been increasing pressure from across the telco and broadcasting sectors for a new regulatory approach, along with alarm expressed at the increased dominance in the market by the pay TV provider and larger broadcaster.
TVNZ remained state-owned, yet it was increasingly using its market power to crowd out other players and was now pushing television content which should be available to all New Zealanders on the free to to air platform behind the pay wall, she said.
Channels planned for Igloo include BBC World News, UKTV, BBC Knowledge, Vibe, Kidzone24, MTV Hits, National Geographic, Animal Planet, Comedy Central, Food TV and TVNZ Heartland. It will also have free-to-air channels.
None of the programming will be in high definition.
Sky Lite
• The Igloo service will offer a set-top box for less than $200 and 11 channels for about $25 a month.v
• Sky has a 51% share of Igloo and TVNZ has 49%.
• The platform is due to start in May or June.
• Igloo will allow downloads of movies and sports for an additional charge.
• The two broadcasters say it will provide more consumer choice but critics say it will entrench their market dominance.
