First, the downhill thing. In Christchurch you have to cycle the mountain all way up yourself to go downhill. Not needed in Queenstown: we have a gondola here!
I grabbed a more appropiate bike and downhilled the fastest and smoothest track, then another time, and another, each time going faster and faster. The entire way down is about 15 minutes. There are also some optional obstacles like very narrow/elevated tracks. Not all of them went very smooth... well, see the video compilation then ;)
It was a shame to wreck a $4500-priced bike in this way. Luckily it was just the front brake handle, and I could continue (slowly) with the remaining brake. After paying $55 at the bike rental shop, I immediately got a brand new bike to follow my way. Great service!