Final plans were for part of the 24 km line to be underground, with a cost of $14.6 billion which would have made it the single biggest transport project in New Zealand history.
[1] Around 2015, Auckland Transport (AT) began investigating light rail as a method of relieving heavy bus congestion on arterial roads.
It was proposed that light rail would start with a Wynyard Quarter to Britomart connection, it would then run along Queen Street and Dominion Road to Mt Roskill before terminating at a depot in Sandringham.
The two transport agencies concluded that an extension of the Onehunga Line to the airport would be too expensive and "poor value for money" when contrasted to light rail.
[10] In the first half of 2017, the NZTA investigated an "Advanced Bus Solution" as an intermediate step to the implementation of the City Centre-Māngere Light Rail line.
The then-transport minister Simon Bridges responded by saying that he didn't think that it was feasible to bring light rail forward to 2021, stating that "We support route protection now, and we want to have a public transport corridor.
It was revealed that the New Zealand Super Fund had already expressed interest in forming an international consortium to design, build, and operate both of the planned lines on the light rail network.
[20][21][22] By January 2022, eventual plans for the City Centre-Māngere line involved a "tunneled light rail" approach, estimated to cost $14.6 billion.
[23] If implemented it would have been the single largest transport project in New Zealand history, beating out the $4.4 billion City Rail Link currently under construction in central Auckland.