Oamaru to Dunedin
The coast road is just outside of Oamaru. Ask locally for directions. This is a steady ride with the sea on your left. The traffic is light and on a good day you can likely spot a few seals or just enjoy long views of sea and sandy beaches. The coast road ends at Herbert so from here the main road takes over heading south. You may want to look at the Moeraki Boulders, these are large, round rock concretions in the sand - they are scattered all down the coast to Katiki Beach.
There is a great tearoom at Moeraki with good coffee and food, souvenirs, and a campground. But Waikouaiti, a small town with campground, pub, and store, is a better place to stop. Why? Because you might want an adventurous overland trip to Middlemarch, the beginning of the Otago Central Rail Trail. However, if you want a city break, head toward Dunedin, 32 km further on.
If you're keen to avoid the main highway you might want to try the inland route from Moeraki to Palmerston. Just out of Moeraki, take a right along Horse Range Road. This is a quiet, scenic, tar sealed route that takes you through farm land and forestry, although it does miss the lovely stretch of coastline between Katiki and Shag Point.
At Palmerston you have the choice to either continue down the coast to Waikouaiti, or alternatively head inland along Highway 85 ("The Pigroot") which rises up the Inch Valley through Kyeburn, meeting the rail trail at Ranfurly and shadowing its progress through to Alexandra.
I love this road, if only because you get a tremendous sense of exhilaration when you get out the other end. It is beautiful, empty, rugged, historic and exhausting. Mostly, you are either pushing uphill or braking down, but the rewards of the landscape - particularly if you reach Middlemarch toward evening - are absolutely magical.
The road is called Ram Rock Road and it runs to the right off the main road just outside of Waikouiti heading south. The gravel road starts out pleasantly enough through farmland but gradually begins to climb steeply before dropping down into a river valley called Buckland's Crossing. It climbs and dips over the Taieri Ridge before coming to Nenthorn, an isolated remnant of old gold towns. Nenthorn once sported several pubs and banks and a population of over two thousand, but is now a forlorn scattering of stone walls and collapsed mineshafts.
If you have time, take a walk down toward the stream and look at the stonework the miners constructed as a foundation for their stamping batteries. A land now given back to hawks, harriers and that immense peace that the Otago landscape seems to breathe in its most isolated moments.
This is a nice alternative if you're heading to Dunedin and want to avoid the main highway. Take a left turn about 3 km out of Waikouaiti and follow the Coast Road through Karitane, continuing on through the sleepy settlements of Seacliff and Omimi and eventually winding your way back to State Highway 1 at Evansdale. This road follows the main trunk line south, with several rail crossings along the way. Once you leave Karitane the road rises sharply and there's a fair bit of up and down, but this is a small price for the magnificent views out to sea and the quiet country roads.
Whether you take the coast road or brave the main highway, there's no avoiding what comes next. Cycle traffic is prohibited on the motorway between Waitati and Dunedin, and the detour takes you up and over Mt Cargill (400m). The gradient is not too bad; find a low gear and the 8 km climb is over before you know it. Once at the top the views over the Otago peninsula are breathtaking and from here on it's all downhill to Dunedin...