The proposed Rockwater Power Project is a hydropower development on the Waitaki River in the South Island of New Zealand. The proposed development will have an installed capacity of 250 MW with a peak flow capacity of 260 m 3/s. The project commences at Lake Waitaki and extends some 35 km downstream on the north bank to a surface power station at Stonewall. Flow is returned to the Waitaki River via an outfall channel.The tunnelling for the project is through Greywacke, which comprises Permian and Triassic age marine sediments that are well indurated and weakly metamorphosed. The rock mass is jointed and sheared and there are frequent faults, which range up to hundreds of metres in width. Three 12 m diameter tunnel boring machines (TBMs) are proposed to excavate the 35 km long tunnel.The paper describes the development of the tunnel alignment, rock mass classification, design of the support, rock and hydraulic loads, support installation options and construction sequencing.The alternatives of rock bolts and shotcrete and segmental lining were both investigated and are discussed. Support using rock bolts and shotcrete offers the potential of rapid construction with minimum support cost when the ground conditions are good, but delays due to support installation increases with deteriorating ground conditions. In contrast, segmentally lined tunnels offer greater certainty in cycle time with reduced program risk. Both options are described and discussed.The horizontal alignment is affected by the presence of major fault zones, which are either avoided, or pretreated by driving conventional tunnels through in advance of the TBM delivery, topographical constraints with the rugged topography and operational hydraulic constraints. Construction requirements, future power station operation and the minimisation of leakage from the tunnel have all impacted on the vertical alignment.The number and sequencing of TBMs to be used is discussed in terms of program time and risk minimisation.