The project’s initial peak capacity was to be 36,000 passengers per hour. However, City Rail Link Ltd (CRLL), the New Zealand government and Auckland council joint-venture company managing the project says that capacity will be reached by 2035, just 11 years after it opens, and is seeking several hundred million dollars to boost maximum capacity to 54,000 passengers per hour.

While the exact extra cost is not yet known, the current projected cost of the tunnel, stations and associated rolling stock is $NZ 3.4bn ($US 2.2bn).

The extra capacity will be achieved by lengthening the platforms of the underground stations to accommodate nine-car trains compared with the original plan to cater for three-car or six-car trains, and adding a second entrance to the new Karangahape station.

Auckland mayor Mr Phil Goff says it will be far cheaper to add the capacity now while the link is under construction than add additional capacity later.
"We've got to future-proof, and it’s a hell of a lot cheaper and less disruptive to do it now than to wait,” Goff says.

Construction is due to be completed in 2024.