Heritage NZ

Heritage NZ: The Otira rail tunnel

From its opening in 1923 the Otira rail tunnel has ranked as an extraordinary feat of engineering. HUGH DE LACY tells its story.

It was the vast quantities of gold being mined on the West Coast from the 1860s onwards that prompted the good burghers of Canterbury to dream of a rail link across the great divide of the Southern Alps.

Otago’s gold had made Dunedin the financial capital of the dominion, and here was all that wealth heading overseas from the West Coast without leaving its benevolent mark on Canterbury.

But it would be half a century, and long after the last of the Coast gold-rushes, before the dream was realised, and then thanks only to the completion of a marvel of civil engineering in the form of the Otira rail tunnel.

A notoriously rugged road for stagecoachs would be hacked in the 1880s over Arthur’s Pass –discovered by the West Coast and Nelson district engineer Arthur Dobson – but railways were the obsession of the Victorians, and the first plans for a coast-to-coast railway were drawn up as early as 1865.

Political and economic considerations delayed a start to the project for 20 years, at which time a London financial syndicate was prevailed upon to put up $5 million for a 378 kilometre line to be built in 10 years. But by the end of that period the Midland Line, as it was called, was still unfinished: The contractor had gone bust, leaving the Government the owner of three tunnels, 82 bridges, 21 stations and partially completed lines on either side of the Alps, but with the critical challenge of the 15 kilometre stretch between Arthurs Pass village and Otira still unmet.

A range of solutions to the traverse of the pass was proposed, including a cog traction system to drag trains over the summit, but it was soon concluded that a long tunnel was the only viable option. The first proposal was for a 10 kilometre-long tunnel at a gradient of 1:37 but the sandstone and shale through which it would have to pass made it uneconomic.

Instead an 8.55 kilometre-long tunnel at the steeper gradient of 1:33 was decided on, to be driven from an altitude of 737 metres at the eastern railhead at Arthurs Pass down to 483 metres at Otira.

The call for tenders did not go out until 1906, and initially met with no response, but eventually a New Zealand company, J. McLean and Sons, was persuaded to pick up the challenge at a contract price of $1.2 million. The contract envisaged the shifting of 240,000 cubic metres of rock and 8100 cubic metres of earth, and the first shot under the drill-and-blast tunnelling technique of the day was fired by Prime Minister Sir Joseph Ward at Otira in August 1908. Work started at the Arthurs Pass end in July the following year.

The tunnelling technique called for compressed air-powered reciprocating and rotary drills making 12 holes in the rockface at a time, each 1.5 metres deep. The four centre holes converged, and the charges, ranging in size from 18kg to 22kg, were fused so that these detonated first, loosening about 11.5 cubic metres of rock and giving a progress rate of about four metres a day.

The compressors were powered by hydro-electric generators sited in waterfalls at either end, driving Pelton wheels connected to dynamos each putting out 433kW at 500 volts of direct current. Winches driven by compressed air and operating through steel wire cables powered trucks to take the spoil from the face to 10-tonne electric locomotives that removed it from the tunnel on 760mm gauge tracks, and also brought in construction materials and provided transport for the workers.

Shale and rotten rock often slowed progress, and the work teams had to deal with water flows that could be as high as 13,500 litres a minute. The project was plagued by industrial disputes which took a sharp edge to them when a 15 metre section at the Arthurs Pass end collapsed, killing one worker and trapping two others for four days.

The cold, the wet and the isolation made the Otira tunnel a hellish worksite, resulting in legendary alcohol consumption and gambling among the workers. Mona Tracy’s 1960 book “West Coast Yesterdays” (A.H. & A.W. Reed) records that one entrepreneur was able to launch a merchandising business on the revenue from the 40 tonnes of empty bottles he repatriated to civilisation, while on one occasion five stagecoaches were held up on the road while a two-up school decided the fate of hundreds of pounds of workers’ wagers.

The hardness of the rock in the tunnel combined with other factors to reduce progress to half the projected rate, and by 1912 only 2.9 kilometres of the tunnel had been completed. The following year the exasperated contractor in effect walked off the job and turned it over to the Government. In 1914 World War I sent labour and materials costs spiralling 60 percent, and labour shortages saw the workforce progressively cut from 240 to 140 men.

Dignitaries assemble at the tunnel break-through ceremony- August 1918

Work continued nonetheless, and in August 1918 the Minister of Works, Sir William Fraser, fired the breakthrough shot and led a contingent of guests through the hole. As a tribute to the professionalism of the surveyors, the two drives were found when they met to be only 28mm out in level, and 19mm out in alignment, and the total length of the tunnel was just 914mm less than had been calculated.


It took another three years to complete the lining of the tunnel – and the Midland Line itself – and in August 1923 both were formally opened. At the time it was built the Otira tunnel was the longest in the British Empire and the seventh longest in the world.

In the absence of a safe harbour on the West Coast, the Midland Line and the tunnel which made it viable have been vital to the region’s development, and no more so than now when millions of tonnes a year of coking coal and dairy products are railed across to Lyttelton for export.


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