[2] The first car to traverse the route from Auckland to New Plymouth seems to have been an 8 hp Cadillac in 1905, though Ōtorohanga to Te Kūiti was by train and, between Awakino and Mokau, a horse assisted on the beach.

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.

Bypassing Waitara, it reaches New Plymouth then turns inland, passing to the east of Mount Taranaki via Inglewood, Stratford and Eltham to Hāwera.

There is a gap in the SH 3 designation here due to the closure of the Manawatū Gorge route in 2017, and traffic between Ashhurst and Woodville is required to detour via nearby Saddle Road while a replacement highway on a new alignment is under construction.

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.

[27] The highway through the Manawatū Gorge between Ashhurst and Woodville in the Manawatū-Whanganui region was closed in April 2017 due to a large slip, with SH 3 traffic diverted via Saddle Road.

Contractors were pulled out of clearing the slip in July 2017 due to ongoing geological movement in the hill, closing the road indefinitely.

[29] Known as Te Ahu a Turanga – Manawatū Tararua Highway, construction of the $620 million road[22] began in January 2021 and is expected be completed in mid 2025.

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.