The project consists of a 3.5 km (2.2 mi) long double-track rail tunnel underneath Auckland's city centre, between Waitematā (Britomart) and Maungawhau (Mount Eden) railway stations.

Two new underground stations will be constructed to serve the city centre: Te Waihorotiu near Aotea Square and Karanga-a-Hape near Karangahape Road.

[3] The increase in rail patronage in Auckland during the early 21st century, particularly after the opening of Britomart Transport Centre in 2003, led to renewed interest in the scheme.

[9] Preliminary stages of construction, including the relocation of stormwater infrastructure and tunnelling in the vicinity of the Commercial Bay redevelopment, began in 2016.

He expressed doubts that the tunnel would ever pay purely from a rail point of view, though he acknowledged that there might be other benefits and wider aspects to take into account.

[12] The 1970s plans envisaged a loop connecting with Newmarket as part of a major rapid transit scheme proposed by Dove-Myer Robinson, mayor of Auckland City at the time.

Auckland Mayor Sir Dove-Myer Robinson noted the central government had just spent $33 million for new Wellington suburban trains (the EM class Ganz-Mavag units) and the overhead railway scheme would "cost considerably less while providing a far greater potential.

[17] In 2004, Auckland City Council prepared preliminary plans for an underground railway connecting Britomart Transport Centre to the Western Line in the vicinity of Mount Eden railway station[18] and incorporating three new stations: near Aotea Square, Karangahape Road and the top of Symonds Street.

Banks noted that it attracted cost-benefit returns much higher than many similar-sized roading projects, and would provide much enhanced, integrated access to the city centre.

[25] Brown also strongly supported the tunnel, and further, a rail connection to Auckland Airport, as part of a package of measures to double public transport patronage within 15 years.

[30] In March 2012, Auckland Council decided to bring forward spending from the 2012–2013 budget, in order to continue progress protecting the eventual route.

$6.3 million was spent on work including geotechnical surveys, utility and building assessments, contaminated site reports and rail operations modelling and $1.7m towards providing a revised business case, requested by the government.

This included designation space for a not previously considered station on the current Western Line, just west of Dominion Road.

In addition, the change would allow Mount Eden station to be connected to the CRL, which previously bypassed it, and would separate the east–west junctions, meaning that rail lines would not need to cross each other.

[37] On 27 January 2016, Prime Minister John Key announced in a speech to the Auckland Chamber of Commerce that central government funding for main works construction of the CRL had been confirmed and this would allow Auckland Council to start to construct the main works from 2018, with central funds guaranteed to flow from 2020.

In July 2018, revised projections by City Rail Link Ltd (CRLL) showed the 36,000 capacity will be reached by 2035 – just 10 years after it opens.

[43][44] These reflected the area's Māori history: Waitematā for Britomart, Te Wai Horotiu for Aotea, Karanga a Hape for Karangahape, and Maungawhau for Mt.

[45][44] Together with Auckland Transport, City Rail Link Ltd submitted these suggested names to the New Zealand Geographic Board Ngā Pou Taunaha o Aotearoa (NZGB) for recognition.

In this regard, Council experts have highlighted that NZ calculation methods use a 30-year cut-off (i.e. for evaluation purposes, the tunnel provides no benefit after 30 years, even though much of Auckland's earlier rail and road infrastructure already serves for much longer than that).

[49] The "City Centre Future Access Study" (CCFAS) was prepared by Auckland Transport and released in December 2012.

The CCFAS analysed a number of different ways of improving access to Auckland's city centre and concluded that the CRL was essential, noting that bus-only investment will provide for short-term benefits but in some cases will be 'worse than doing nothing' for private vehicle travel times in the longer term.

[4] In September 2016, the government formally confirmed its intention to fund its proposed share of 50% of the City Rail Link.

Brown and Simpson also confirmed that there would be a total of 96 days in Auckland without an operational train service over the next 13 months, with closures occurring during holidays, weekends and evenings.

[76] It is expected that 2 million tonnes of spoil will be dug out from 2020 and it has been proposed to use it to double the single track section of the North Island Main Trunk line across Whangamarino wetland.

The work started in October 2015 with the relocation of a major stormwater line in Albert St between Swanson and Wellesley Sts.

[79] Construction of these sections of the city rail link tunnels will coincide with Precinct Properties redevelopment of the Downtown Shopping Centre site, due to open by mid-2019.

Auckland Council and proprietors Precinct Properties struck a deal to include tunnels for the City Rail Link directly underneath the premises.

[96] While the train network for the period following the completion of the CRL has not been officially confirmed, Auckland Transport has released a preliminary plan.

[97] This operating pattern was first hinted at in early 2022 in the Auckland Light Rail group's Indicative Business Case appendices, which showed a proposed but discarded Airport heavy rail option where half of all Western Line services diverted from Avondale to Auckland Airport via Onehunga.

A public opinion poll published on 27 June 2012 found 63% of Aucklanders surveyed are in favour of the tunnel, 29% were against it and 8% didn't care.