The aircraft was being flown on the final phase of the student's multi-engine endorsement training. A general handling period was completed before arriving at Camden to carry out normal and asymmetric circuit procedures. The instructor pilot reported that the first approach was normal but the aircraft touched down firmly on the mainwheels, followed by a firm touchdown on the nosewheel. A touch-and-go was carried out and the landing gear retracted normally after take off. On downwind, during the next circuit, the landing gear extended normally with three green landing gear locked indications. The nose gear was also confirmed to be extended, using the mirror on the right engine nacelle. Both pilots again confirmed three greens during the turn onto final. The touchdown was smooth but the nose gear collapsed and the aircraft slid to a halt on its nose. The repair agency later reported that the nose gear downlock was found out of rig to the extent that the hydraulic pump continued cycling with the gear down. It is probable that this condition allowed the nose gear to unlock when it took the weight of the aircraft during landing.