Auckland Transport is backing a finding that an airport rail loop would provide higher long-term benefits than other transport options for the region's southwestern sector.
The board of the council-controlled organisation yesterday gave its approval for a multi-agency study to progress to a detailed scheme assessment and business case for providing airport rail links from both central Auckland and Manukau.
These are costed in the draft Auckland Plan at $1.18 billion and could not be built until a $2.4 billion central city tunnel, which the Government has yet to be persuaded to support, provides the dead-end Britomart station with much-needed extra capacity.
The 30-year plan does not envisage completing an airport rail loop until 2040 - despite an election promise by Auckland Mayor Len Brown to provide one by about 2020.
But a report from the southwestern sector study - which was conducted for the Transport Agency, KiwiRail and the airport company as well as Auckland Transport and its council parent - says planning provisions for future links must be made soon.
"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."
* Includes links from Onehunga to Puhinui-Wiri.
* Predicted cost - $1.18 billion.