Agency regional director Ernst Zollner confirmed further investigations for rapid transit connecting the city to Mangere would be limited to light rail or a busway.
"This is based on evidence from Auckland Transport that a heavy rail option to the airport would present poor value for money."
The same report is to be presented to the AT board today in a session closed to the public. The meeting's agenda lists the reason for privacy as commercial sensitivity.
An AT spokeswoman expected the report to be made publicly available shortly after the meeting.
Campaign for Better Transport convenor Cameron Pitches, who has been pushing for rail to the airport for more than a decade, said he has now given up on it ever happening.
He said it was the only sensible option and did not think the business case for trams took into account the cost for new stations and works necessary to complete the network.
The addition of a busway as an option was also new to Mr Pitches, who said the project needed greater transparency.
"It's all very mysterious how transport decisions get made."
Auckland Council infrastructure chairman Mike Lee, who heads a steering group of parties including KiwiRail considering rapid transit options for the airport, said he believed the imminent removal of the Neilson St overbridge at Onehunga would compromise the possibility of protecting the route for rail.
Removing the overpass was extremely short-sighted, he said, because a rail overbridge would need to then be built to allow for commuter rail, adding significant costs.
"The other odious aspect to this is NZTA suggesting they support light rail to the airport and foreclosing on conventional rail while conspicuously refusing to endorse light rail on the isthmus. Auckland taxpayers and ratepayers deserve better than this nonsense."
The transport bodies are under pressure from Auckland Airport to make a decision by August when the company needs to know what form of rapid transit to include in its plans to build a new domestic terminal to by 2021 and a second runway a few years later.
Today Prime Minister John Key said he was not surprised at the decision to favour a different - and cheaper - form of mass transit to and from the city's airport.
"It's hardly surprising. Rail to the airport would have been extremely expensive and difficult to implement so I would have thought the right decision has been made that other alternatives should be explored."
Mr Key, who also holds the Tourism portfolio, said while many large international cities had a direct city-to-airport rail link many were historic and not retrofitted.
"As we know with rail sometimes it can prove to be very successful and we're seeing increased patronage on the new electrified and double-tracked lines but establishing that to the airport, I would have thought, would be a very expensive exercise."
