Start your ride from Huka Falls and bike along the Waikato river to Aratiatia Dam and back to Huka Falls. It's a perfect loop that is about 14-16 km. (depending on your tracker) It is possible to go back the same way, but I reckon most people bike this track anti-clockwise. So be careful, people do walk along the bike track and watch out for bikers coming the other way.
The trail is a grade 2, but I would say some parts are more towards a Grade 3.
The first part of the trail has more climbs than the rest of the trail.
After you pass the bridge/state highway 1, there are a few climbs again, but after that, it will mostly be downhill and flat until you reach the Aratiatia Dam.
Follow the signs with blue arrows.
The flood gates for the dam opens at 10 am, 12 pm, 2 pm and 4 pm during summer.
So try to time your bike ride if possible so you can see the flood gates open.
"The tiers of the rapids and steep walls of the gorge are remnants of an ancient volcano. It is said the name "Aratiatia" comes from the meaning "stairway of Tia". Tia was a leader from the waka Te Arawa, and passed through the gorge with a small party on route to discovering Lake Taupō."
Read more about the Aratiatia Rapids here:
After Aratiatia Dam (if you're going anti-clockwise) it will be a flat ride back to the bridge/state highway 1.
After the bridge, there will be a few climbs again, but they are not too steep and you will ride through native forest and see geothermal streams.
The bike ride takes approximately 1-3 hours
Have fun and enjoy