Main route

The highway leaves SH1 at Washdyke, an industrial suburb of Timaru, travelling initially northwest through Pleasant Point then continuing to the town of Fairlie. From here the route tends westward and rapidly increases in altitude, passing the southern end of the two great Mackenzie Basin lakes of Tekapo and Pukaki.

From Pukaki the highway turns southwest across the upper reaches of the Waitaki Valley, passing through the former hydroelectricity service town of Twizel and Omarama before again climbing to cross the Lindis Pass, which, at 971 metres, is the highest point on the highway's path. The highway winds along the valley of the Lindis River, reaching the upper Clutha River not far from Tarras.

From here, the highway runs close to the eastern shore of the man-made Lake Dunstan before passing the historic town of Cromwell. The highway turns south as it travels through the Cromwell Gorge passing the country's third largest^{[citation needed]} hydroelectric dam, the Clyde Dam at Clyde. The highway continues along the Clutha Valley through the town of Alexandra and past Lake Roxburgh to the town of Roxburgh. Much of this part of the highway's journey is past fruit orchards, and historic gold-diggings from the Central Otago Gold Rush.

From Roxburgh, the highway turns southeastward, passing Ettrick, Raes Junction and Beaumont, where it turns away from the Clutha, before reaching Lawrence, close to the original strike in the Central Otago Gold Rush. From here, the highway continues past Waitahuna and the twisting Manuka Gorge, reaching the plain of the Tokomairaro River at Clarksville, three kilometres south of Milton.

Spur sections

State Highway 8 has two spurs: State Highway 8A and State Highway 8B. These two spurs connect the highway with State Highway 6, which lies on the western bank of Lake Dunstan. State Highway 8A runs for 23 km from SH 6 at Luggate to connect with SH8 close to Tarras. State Highway 8B runs for just 2.6 km from SH8 at Deadmans Point across the lake to the town of Cromwell. SH 8B forms part of the main route between Queenstown and Christchurch.

Route changes

The course of SH8 between Clyde and Tarras was substantially altered during the 1980s as part of the construction of the Clyde Dam, which flooded much of the Cromwell Gorge, including the former route of the highway. A completely new section was built between Clyde and Cromwell; for SH8B the old single-lane bridge crossing the Clutha was replaced with a new two-lane bridge approximately 1.5 km to the north.