A pivotal moment came in 2021 with the assembly and launch of the tunnel boring machine (TBM), Hiwa i te Rangi, at Māngere. This highly specialised machine began its journey north, excavating the tunnel while simultaneously installing precast lining segments to form the tunnel walls. Crews worked in continuous shifts to maintain progress, managing complex logistics such as spoil removal, segment delivery, and machine maintenance. The transition from surface construction to full-scale underground tunnelling marked a major step forward for the project.