The Hapuawhenua Viaduct is situated in the North Island's Central Plateau just northwest of Ohakune. It was constructed as part of the North Island Main Trunk (NIMT) in 1908 and was a functioning part of that railway for the subsequent 80 years. The Hapuawhenua Viaduct is one of a pair of large curved steel truss railway viaducts, which is a unique occurrence within New Zealand.

The Hapuawhenua Viaduct was completed in 1908 and was one of a concentration of five viaducts that were built between Ohakune and Erua; a series that was necessary to navigate the difficult terrain which stood in the way of the completion of the NIMT. All of these structures were designed by the notable Public Works Department (PWD) engineer, Peter Seton Hay. Both the Hapuawhenua Viaduct and its close neighbour, the Taonui Viaduct, were constructed under the 'co-operative system' and the steelwork was manufactured at the PWD workshop at Mangaonoho, which was situated between Hunterville and Taihape and in operation from 1898. The workshop ran at full capacity in order to facilitate the construction of these viaducts which was a central reason why the other three viaducts in the crucial last section of the NIMT were contracted out to a private firm.

In 1906 the southern railhead had reached Ohakune which meant that the fabric for the viaduct travelled over the completed line for most of its journey, but a temporary line had to be built in order for it to then be transported to the Hapuawhenua site. There was also a service road built in 1906 which later became the road that coaches transported train passengers along between the railheads. The close proximity of the base of NIMT southern construction at Ohakune allowed the Resident Engineer, Frederick William Furkert (1876-1949), to monitor the progress of the co-operative system workers. Furkert kept a close eye on what was happening and made frequent visits to the Hapuawhenua site during construction as is evidenced by the fact that the various phases of the viaduct's construction were photographically documented by Furkert.

Unlike the other Hay designed viaducts on the NIMT, the Hapuawhenua and Taonui Viaducts are both rare examples of curved structures each being built with a 201m radius. Curved viaducts were first built for railways and came out of the railway boom of the mid-late 19th century. Despite the aesthetic appeal of curved viaducts, bridges are primarily meant to be functional structures and therefore curved examples are not common, as it was and still is more practical to build straight bridges because they are typically easier to construct and comparatively cost less to maintain over time. Within New Zealand there is only a small group of large curved bridges and viaducts, and the Hapuawhenua Viaduct is among the oldest, and longest at 284m, and its 45m height also ranks it among the tallest of this group.

By August 1907 the preparation of the Hapuawhenua site was completed, the labourer's camps were set up on the southern side of the gully, and excavations for the viaduct's pier footings were advanced. From then work progressed steadily, which meant that the abutments and 13 mass concrete flanking piers were completed by December 1907. While these were being constructed, work also began on the four central lattice steel piers. However, the curve of the Hapuawhenua Viaduct meant that these and the girders were constructed in a different sequence to contemporary straight NIMT viaducts.

The straight viaducts were generally built one pier at a time from edges inwards with the girders being placed as each couplet of piers was completed. However, the Hapuawhenua Viaduct's piers were built in a synchronised fashion, one tier at a time from the ground up using a series of guy lines attached to hoisting machines, called derricks, and then all the girders were raised into position last. Therefore, instead of construction being aided by being able to progressively walk further out onto the viaduct throughout construction, access to the pier heads at Hapuawhenua was via very tall ladders. This construction technique also meant that there was a heightened level of accuracy and precision needed in order to secure the components of each pier on the correct alignment and in manipulating the hoisting system. However, this finesse was combined with a certain amount of brute strength. For example, the pier head girders weighed about three and a half tons, and the truss girder approximately nine and half tons, and these were positioned using the manually operated derricks. Once the piers were capped with their steel plate pier heads, five 19.5m lattice trusses were positioned between them, and then the finishing touches were added to the structure so that it was finished in April 1908.

It was then a matter of laying track across the viaduct and finishing other structures and earthworks in the last section to facilitate the first through train trip from Wellington to Auckland, the 'Parliamentary Special,' in early August 1908. However, Premier Joseph Ward was able to experience the viaduct prior to participation in that historic event because he visited Ohakune in mid-June at which time he was taken on a train excursion over the structure. The Hapuawhenua Viaduct was then the second longest viaduct on the NIMT, but because the Mangaweka Viaduct was demolished in the early 1980s, it is now the longest NIMT construction-era viaduct.

It is a tribute to the skill of Hay as a designer, and the technical skill and capabilities of those manufacturing the components for the viaduct and the labourers who built it, that minimal alteration was needed during the 80 years it was in use. The most significant changes occurred as part of the NIMT wide strengthening programmes between 1925 and 1934, and again in 1971. The structures was also painted in 1964, and late in that decade telephone wire insulators were added to the western side of the viaduct.

As early as the mid-1960s there had been calls to bypass the Hapuawhenua and Taonui Viaducts for the good of the smooth and efficient operating on the NIMT. The main reason cited for this was that since the 1930s there had been a speed restriction of 20mph (32kph) on each of the structures due to their small radiuses, and a deviation would also reduce maintenance costs, as well as the risk of derailment. An added incentive for deviation was that the Hapuawhenua Viaduct required costly underpinning if it was to continue to be viable within the functioning railway. Therefore the Horopito Deviation, completed in 1987, was a practical and logical solution. Included in the bypass were two replacement viaducts for the NIMT construction-era Taonui and Hapuawhenua Viaducts. The Hapuawhenua's replacement in the functioning NIMT is close by, is also a curved viaduct, and has been described as 'an elegant and impressive study in reinforced concrete.'

When the deviation was opened for traffic in 1987 the Hapuawhenua Viaduct was rendered obsolete and its rails were removed. However, because it was still publically accessible a walkway was created down the centre of the deck using old sleepers and was flanked by a new wooden handrail. As part of the planning for the deviation possible uses for the NIMT construction-era structures were considered, and it was decided that they would be retained and placed under DoC management to eventually be incorporated into a walking track. The land immediately surrounding the viaducts had become part of the Tongariro National Park in 1916. Therefore, a like for like swap of land between DoC and the Railways Department was a logical arrangement when plans for the deviation were being devised. It was anticipated that the combination of these important structures and the natural features along the walking track would make it an appealing tourist attraction.

In early 2009 a conservation programme at the Hapuawhenua Viaduct was completed. In the period between its redundancy from the functioning railway and its reopening on 14 February 2009, the viaduct had been mostly unused aside from a brief stint as the platform for a bungy jumping operation in 1988. This very short lived AJ Hackett enterprise is reportedly the first commercial bungy jumping operation in New Zealand and the world. The installation of new decking and the conservation of the structure have meant that visitors using the Ohakune to Horopito Coach Road walking track can also walk over the Hapuawhenua Viaduct.

The Hapuawhenua Viaduct was an integral component in the push to finish the NIMT after 23 years of construction, and then the revolutionary effects that the railway created in regards to transport and New Zealand's economy. The longevity of the structure can be attributed to the technical skill of those involved in designing, manufacturing the fabric for, and constructing the viaduct. The extended length of the curved form of the viaduct creates an impressive impact and contributed to it being an aesthetic highlight of the NIMT, and subsequently the Tongariro National Park. This viaduct also gains historical significance because the railway's deviation away from the structure was a direct result of the electrification of the NIMT, which was a defining moment in New Zealand's rail history. The viaduct's subsequent retention and inclusion in the Tongariro National Park can be seen as evidence of the increased awareness of New Zealand's rail and engineering heritage.