The Macaulay Valley is a huge glacial valley with towering mountains on either side and a broad braided gravel floor.
The first thing you have to do on this adventure is cross the Macaulay River and that's dangerous in spring and after heavy rain. But in late summer the river is generally only knee deep and not swift enough to wash away your bike if you hang on tight.
A four wheel drive track goes 19km (12 miles) up the valley to New Zealand's best alpine hut which is a popular base for deer hunters and mountain climbers, not to mention quad bikers, four wheel drivers and their families.
The trail surface is coarse gravel - bouldery in places. It's very jittery all the way and there are a dozen or so creek crossings deep enough to wet your feet - same on the way back! This adventure is about the awe of the rugged alpine environment rather than the cycling pleasure.
A similar trail that is more challenging and more stunning starts at the same place and goes 27km up the Godley River. The two trails can be added together and combined with some walking into a challenging two or more day alpine adventure.