State Highway 3 (SH 3) is one of New Zealand's eight national state highways. It serves the west coast of the country's North Island and forms a link between State Highway 1 and State Highway 2. Distances are measured from north to south.
For most of its length SH 3 is a two-lane single carriageway, with at-grade intersections and property accesses, both in rural and urban areas.
An 1871 account of a coach journey from New Plymouth to Wellington described the very poor road round the coast via à Âpunake to Whanganui and the much better and well used road south from there. A government subsidy of ã3,000 (2016 equivalent $400,000) a year was being paid for the coach north of Whanganui.
A Mokau â Awakino horse track was widened to a dray track about 1897. It was then possible for a horse and buggy to cover the â now 162 km â from New Plymouth to Te Kà «iti in 17ý hours. The first car to traverse the route from Auckland to New Plymouth seems to have been an 8 hp Cadillac in 1905, though à Âtorohanga to Te Kà «iti was by train and, between Awakino and Mokau, a horse assisted on the beach.
A 1910 Te Kà «iti meeting called for metalling of the road to Awakino. Mount Messenger Tunnel opened in 1916 and its single lane was enlarged about 1983. ã3,000 (2016 equivalent $280,000) was provided in 1919 for the Awakino Gorge section, including the tunnel (enlarged in 2011), and it was opened in March 1923 at a total cost of about ã60,000 (2016 equivalent $5.8m). Most of the route was gazetted as a government main highway in 1924, some sections were tar-sealed in 1925 and more work had been done on the road by 1936, leaving only a few mud sections. In 1937 the between à Âhaupà  and Te Awamutu was the only unsealed section north of Tokanui. By 1949 the road was sealed as far south as Te Kà «iti and the reinforced concrete bridge over the Waipàat à Âtorohanga had been started.
The wide, single-lane, 11-span, , steel, Mokau Bridge opened in October 1927 (official opening 17 December), replacing a punt. The cost was ã35,000 (2016 equivalent $3.3m). It had a lifting span allowing passage of vessels up to wide and with a clearance at high tide. In 2001 the 1927 bridge was replaced by the current 9-span, , double lane, pre-stressed concrete bridge for $6.2m (2016 equivalent $8.5m). The current bridge does not allow for shipping, as the first ship to pass under the old lifting span was withdrawn three months after the bridge opened.
The highway leaves SH 1C at Hamilton, and initially heads south through farmland, passing through Te Awamutu. Sections of this stretch of the road are four-laned, but not divided carriageway. From à Âtorohanga it follows the valley of the WaipàRiver to Te Kà «iti. The road then turns southwest through sparsely settled farmland, crossing several ridges before following the Awakino gorge to reach the coast, which it follows around the North Taranaki Bight. At the Tongaporutu River the highway turns inland to avoid coastal cliffs and climbs over Mount Messenger before descending via the Mimi River valley to return to the coast near Urenui. Bypassing Waitara, it reaches New Plymouth then turns inland, passing to the east of Mount Taranaki via Inglewood, Stratford and Eltham to HÃÂwera.
From HÃÂwera the highway follows the coast of the South Taranaki Bight south-eastwards to PÃÂtea and Whanganui before leaving the coast and heading to Bulls. Here, SH 1 from the north intersects with SH 3, and the two highways are concurrent for the next to Sanson where SH 1 diverges to the south. SH 3 continues its south-eastern route to Palmerston North, where it runs through the city streets and changes to a north-eastern direction near the city centre. It maintains this direction as far as Ashhurst, where it crosses the Manawatà « River and intersects with SH 57. The Te Ahu a Turanga â Manawatà « Tararua Highway which replaces the former Manawatà « Gorge route opened to the public on 11 June 2025. SH 3 enters Woodville and terminates at a junction with SH 2.
The Awakino gorge is prone to closure by slips and rockslides in heavy rain.
Construction of a bypass of Bell Block, a satellite town of New Plymouth, was completed in 2010. The bypass is a four-lane, divided expressway with an interchange at Henwood Road providing access to Bell Block.
In 2016, a section of SH 3 in Taranaki was realigned replacing the "deadly" Normanby overbridge.
State Highway 3 will form part of the future/proposed Southern Links motorway project in the South/West of Hamilton and Tamahere.
In January 2016, funding was announced for a road bypass of the tunnel north of New Plymouth. Construction of the Mount Messenger bypass was expected to begin within two years. On 24 August 2020 Te Korowai Tiahi o Te Hauauru, whose rohe the road crosses, withdrew support from the bypass roading project while in High Court. The Poutama Charitable Trust and New Plymouth locals protested against the project while court was in session, with the argument that this new route would destroy the valley's ecosystem. However, the Transport Agency responded by saying that the bypass is the least ecologically destructive and will deliver road safety. Construction of the Mount Messenger bypass, dubbed Te Ara o Te Ata, began on 6 October 2022.
SH 3 has one spur, designated State Highway 3A. This 15.6 km stretch links State Highway 3 just west of Waitara to Inglewood, providing a shortcut and bypass of New Plymouth. The entire length of highway carries the name Mountain Road with the sole exception being a very short link of Rimu Street in Inglewood to rejoin SH 3.