These delays meant that it wasn't until 15 September 1924 that a start was made on driving the tunnel from the surge chamber already excavated at the downstream end. The initial average speed of cutting through the papa rock (a Māori-derived term for blue-grey mudstone), was one metre per hour with a maximum advance of 1.5 metres per hour, including all stops. Unfortunately, large boulders were soon encountered, sometimes every 1.5 metres, which all had to be excavated by hand. The largest boulder was 7.6 m long.
