Poro-O-Tarao railway station

The tunnel to the south of the station was 53 ch (3,500 ft; 1,100 m) long.[2] The Public Works Department awarded a contract to J. J. O'Brien for the Poro-O-Tarao section (including tunnel) in August 1885.[2] The PWD originally required an impossible contract time of two years, and also that it be built before the railway reached Te Kuiti (the railhead did not reach the site until 1897) so materials had to be taken over 46 miles (74 km) of roadless wilderness. Richard Seddon later said that if the line had been built to the site first, the tunnel could have been built more cheaply and easily.[6]

Work began on the tunnel in 1886 and was complete by 27 August 1890.[2]

1886 construction camp at Poro-O-Tarau

Te Ihingārangi had been promised the railway would go around Tihikārearea, home to 23 sacred healing trees and other tapu sites. Local hapū were only alerted to the Government's plans to construct the railway directly through Tihikārearea when construction workers arrived to begin work on the tunnel, around 1886. They told the workers not to put the track through the sacred ground, and tapu poles were put in the railway's path by tohunga. However, the sacred grove was felled before tohunga could remove the tapu, and was said to have caused people to die of illnesses that traditional Māori medicine could have cured. The trees were used for railway sleepers. Also, construction polluted a puna wai tapu (holy spring) where tohunga lived.[2]

In 1935 the driver and fireman of a northbound goods train were overcome by fumes in the tunnel and the Class A^A locomotive crashed and overturned at the station, along with 12 of the train's 46 trucks. Several cattle and sheep were killed, but there were no other casualties.[7]

Poro-O-Tarao was one of the smallest tunnels on the NIMT. As a result, prior to 1920, it had had its invert lowered to allow larger rolling stock. The tunnel is in weak mudstones with considerable ground water. Steel props supported the base of the walls above the lowered invert. Failures of these props and the tunnel's drainage resulted in areas of significant inward cracking of the brickwork. In 1934 a 45 m (148 ft) bulging length was rebuilt to the original profile with a reinforced concrete invert. In 1965 a further 12 m (39 ft) was replaced to improve clearance and to investigate the feasibility of enlarging the old tunnel. The proposal was abandoned as it would have taken at least 9 years and cost twice as much.[8]

Thus, when the railway was electrified, a new 6 m (20 ft) diameter,[9] 1,272 m (4,173 ft) tunnel was built to the west.[8] It goes through Mahoenui Group soft sedimentary rocks, including landslides; the cause of hummocky ground and many small scarps. A drive at the north portal showed gently-dipping massive interbedded weak claystone, siltstone and sandstone. Bedding planes and joints weaken the rock, which is weak anyway. Narrow zones of crushed rock allow water in. The southern approaches to the new tunnel are largely unstable landslide debris, formed of silty clay with rock fragments.[10]

Towards the end of the construction of the new tunnel, it was feared the old one might collapse. It was shrinking 2-3mm a year when tunnel drainage was working and effective strutting was in place, and 8-10mm per year when it wasn't. The central invert drain had blocked and some steel props were buckling. It was so bad, a contingency plan for reopening the old tunnel in the event of it collapsing, was designed. Fortunately it wasn't needed.[8]

Porootarau Tunnel northern portal and station site (to left) in 2018

• 2016 video of electric freight train leaving the tunnel