"There is an urgent need to clarify the way forward and protect possible transport routes because of the pace of land development in the area," it said.
"A preferred multimodal transport alignment needs to be future-proofed early to protect against further land development and encroachment."
It warned the study partners that several private and public plan change proposals for land use developments in the area were already going through statutory processes.
The study report said a rail connection between the airport and the main trunk line near Wiri would be the most economically efficient of seven packages of assessed transport options.
But a full loop, including double-tracking the Onehunga branch line and extending it to the airport, would provide the best network resilience and highest benefits.
Other assessed options included a busway and a light-rail system but the study concluded the high costs of building these would be rewarded by only medium benefits.
Auckland Transport strategy and planning chief Peter Clark said the study forecast airport rail patronage by 2041 at 5500 passenger trips during two-hour travel peaks, in trains running every 15 minutes.
That compares with existing peak capacity of 480 bus passengers.
Board member Mike Williams said huge growth was occurring on routes to the airport.
"There is going to have to be something done about transport in that area, whether or not it is rail - pretty soon we are going to have to put some lines on maps."
Connection
* Includes links from Onehunga to Puhinui-Wiri.
* Predicted cost - $1.18 billion.
