SH 31 route begins at SH 3 in Ōtorohanga. The first section of this route is a joint designation with SH 39. SH 31/SH 39 initially travels north-west along Kawhia Road. After 14 km and just after the locality of Tihiroa the highway meets a junction where SH 39 carries on north and SH 31 turns sharply west, staying on Kawhia Road. The road from here on is extremely windy and treachourous, having in 2012 been assessed as the state highway with the highest personal risk.[1] SH 31 eventually reaches Pouewe Street in Kawhia where it terminates.
It was classified as a state highway on 1 April 1948.[2]
In 1885 the road crossing the river was only 6 ft (2 m) wide at Ōpārau.[3] From about 1900 a coach ran for passengers between Kawhia and Te Awamutu,[4] with a launch between Kawhia and Ōpārau.[5] Ōpārau Ferry Bridge opened in 1913.[6] In 1924 a 52 ft (16 m) concrete bridge was built over the Ōpārau to carry what is now SH 31.[7]Metalling of the road was completed in 1926.[8]
