New Zealand's transport minister Mr Simon Bridges says more accurate cost estimates will emerge as planning and design progresses and project needs are refined, which is likely to affect final cost estimates. However, Auckland Mayor Len Brown is sticking to the $NZ 2.5bn estimate and says the business case is consistent with that figure. "Auckland Transport advises that no new information has arrived, no recalculation has been made and expectations of final costs have not changed."

The 3.4km underground rail loop will link Britomart station with the Western Line at Mount Eden via two new underground stations at Albert Street and Karangahape Road.

Preliminary construction started in June with major works expected to commence in 2018.

A contract between the New Zealand government and Auckland Council is being negotiated, setting out how projects costs will be shared, timing of payments and how the rail link will be owned and managed.