It's about 4-5 hours DOC time return trip from Avoca Homestead. A fit person can do it in 2.5 hours. Just follow Broken River. I counted 13 compulsory river crossings - they're all about the same difficulty, so if you get across the first couple of times, you should be fine for the whole trip. I did it when the "Waimakariri River at Below Otarama" flow on the ECan website was 55 m³/s, and it was okay. I think it may be acceptable up to 70 m³/s, but I wouldn't go if it was any higher than that. The river is pretty swift.