Its main destination is Hamilton Airport and Mystery Creek, where the National Agricultural Fieldays are held.

SH 21 was gazetted as a brand new state highway designation in 1997.

[1] SH 21 is actually 120 km (75 mi) south of where the number is supposed to be when considering the general pattern of assigning increasing state highway numbers in a north–south direction; in reality, it should lie somewhere in South Auckland.

[citation needed] The first bridge opened on 12 February 1879,[2] costing £999 17s 4d, with a £300 government subsidy,[3] £90 each from Waipa and Waikato County Councils and the rest paid by donations,[4][5] paid by a long list of subscribers from 1878.

[10] It is 1.78 km (1.11 mi) from the Tamahere end of SH21[11] with a road width of 20 ft (6.1 m) wide, plus a 5 ft (1.5 m) footpath, which cost £5,660[12] (£3,000 from government and the rest shared by the counties)[13] and was designed by Jones and Adams (see also Tuakau Bridge).