This is a 112km (70 mile) remote back-country cycle touring route mainly on a gravel road.. It is normally only open from Boxing Day to Easter Monday.
Here is a description of the route taken from http://www.nzcycletrail.com/rainbow-trail...
"The Rainbow Trail starts approximately 9 km east of St Arnaud at the Rainbow Road turn-off on State Highway 63. The trail starts with a sealed section of road that winds its way to the Old Rainbow Homestead and toll gate where you will pay the $2 fee during the open season. Permission for access is required outside the open season.
"Beyond the old Rainbow Homestead and toll gate the Rainbow experience really begins. The trail fords a number of streams and crosses a bridge over the Wairau River before entering 'Hell's Gate', a scenic narrow gorge. You'll find the Coldwater Creek campsite just above the gorge and the boundary between Rainbow and Molesworth stations is just a little further on from there.
"The turn off to the Sedgemere Sleepout is located just after the trail recrosses the Wairau - it's a few hundred metres down a side road off the trail. A short detour past the sleepout carpark will provide you with a spectacular view over several high country lakes.
"Island Saddle marks the highest point on this route and the boundary between Marlborough and Canterbury. Not far from Island Saddle, a further detour (3 km) will take you to Lake Tennyson, a scenic lake formed by a glacial moraine dam - and an ideal spot for a picnic.
"Approaching Hanmer Springs there is a 3-way intersection at the top of Jacks Pass. If you're keen for more cycling, continue left here on to the Molesworth Muster Trail, a 207 km cycle trail through Molesworth Station to Blenheim. Though you might want to stop in Hanmer to restock supplies first.
"Heading right down Clarence Valley Road, you'll enjoy a long downhill into the Hanmer Springs township and a soak in the famous hot thermal springs."
The route is used for a celebrated annual mountain bike race called the Rainbow Rage. http://www.rainbowrage.co.nz. The photos on Singletracks were taken during this event a few years ago.