The aircraft was being taxied across another runway towards a taxiway. Approaching the edge of the flight strip, the instructor noticed a 20 centimetre deep spoon drain in the path of the aircraft. He took control and attempted to avoid the drain but the nosewheel entered the drain and broke off. The presence of the drain was indicated on a diagram of hazardous taxying areas available to pilots but the area was not marked by cones. The aerodrome was being used, for taxiing purposes, as an all-over field, contrary to approved procedures. The pilot saw the drain obstruction, which was situated just outside the flight strip, too late to take effective avoiding action.