The cost of building the Auckland City Rail Link, one of the largest transport infrastructure projects in New Zealand, has increased by around NZD1bn ($670m) to NZD4.419bn ($2.97bn).

City Rail Link (CRL), the company responsible for delivering the project, announced the revised cost. The estimate has been submitted to the project’s sponsors, the government and Auckland Council for approval.

CRL chief executive Dr Sean Sweeney said that the revised cost has been prepared following a comprehensive review of project costs.

Sweeney stated: “The NZD1bn cost increase on the previous NZD3.4bn estimate made in 2014 reflects significant changes impacting the project in the past five years.

“No one could have foreseen the competitive pressures that have occurred in the construction industry over the past few years and the impact that has on costs.”

“No one could have foreseen the competitive pressures that have occurred in the construction industry over the past few years and the impact that has on costs, particularly for a project the scale and complexity of the City Rail Link.”

The increased cost is attributed to multiple reasons, including the expansion of the scope, construction inflation costs and an increase in the contingency risk allowance.

The project sponsors are expected to announce their decision on the revised estimate early next month.

CRL also announced the Link Alliance as the preferred bidder for its substantive C3 contract, the Auckland City Rail Link’s largest package of work.

Link Alliance includes Vinci Construction Grands Projet, Downer NZ, Soletanche Bachy International NZ, WSP Opus (NZ), AECOM New Zealand and Tonkin + Taylor.

The scope of the contract includes building underground tunnels and stations, as well as an NZD75m ($50.39m) early works contract.

The Auckland City Rail Link is an under-construction 3.45km-long twin-tunnel underground rail link between Britomart Station and Mt Eden Station. The project is slated for completion in 2024.