Myself and 6 friends were tramping locally after getting permission of a local farmer. We were exploring the headwaters of the Ngunguru river. The terrain was steep in places. I began following the steep top edge of a stream bed a long way back from the edge but in thick knee deep vegetation. I suddenly fell sideways 3 meters into what was a sinkhole or cut in from the stream, landing on my back onto a large rock. Extremely painful. I crawled along the stream bed in agony to more open ground where my friend could reach me. I got someone to activate my PLB. I was immobile in agony. I was eventually located by helicopter, a stretcher lowered, winched up into it an d taken to Whangarei Hospital. I had broken my back Lumbar 1 vertebrae. After 3 months recovery still having trouble with back.