A Reims Cessna F406 Caravan II aircraft, registered ZK-VAF, was being operated on a passenger charter flight from Darwin to Tindal, Australia. At approximately 85-90 kts during the take-off roll, the nose landing gear (NLG) collapsed. The aircraft slid to a stop, the pilot shutdown the engines and all occupants evacuated the aircraft uninjured. Prior to this occurrence, on 2 and 19 June 2003, pilots reported difficulties obtaining a ‘down and locked’ indication for the NLG. Maintenance actions rectified the problems at that time. An examination of the aircraft following the NLG collapse revealed that no damage was evident to any NLG components, or the NLG attachment structure. The NLG rigging was checked and reported to be within tolerances. Damage to the aircraft included abrasion damage to the lower forward fuselage and NLG doors. Both propellers were substantially damaged from ground contact. The NLG hydraulic actuator was removed from the aircraft for further examination by the ATSB and was taken to a specialist hydraulic facility for functional testing prior to disassembly. The actuator passed all required functional tests, however, it was noted that the integral microswitch had been incorrectly adjusted to the point that it did not obtain switchover during operation of the hydraulic actuator. The microswitch was effectively always providing a signal indicating that the actuator was ‘down and locked’. However, as the actuator microswitch was wired in series with the NLG overcentre microswitch on the aircraft, the landing gear indications would have appeared normal. The aircraft landing gear hydraulic system was powered during landing gear extension, however hydraulic power was removed once all three landing gear downlock microswitches were activated. Advice from the aircraft manufacturer confirmed that the installation of the incorrect NLG actuator rod-end locking device, combined with a incorrectly adjusted NLG actuator microswitch, could lead to a NLG collapse if the dynamic loads experienced during take-off overcame the overcentre mechanism of the NLG drag brace assembly. Alternatively, the hydraulic landing gear system power is removed once all three landing gear downlock microswitches are activated. It is also possible that, with the nose landing gear actuator microswitch incorrectly adjusted to the `down and locked’ position, hydraulic power was removed from the landing gear system after activation of the two main landing gear down lock microswitches. If this occurred prior to the NLG actuator internal locking devices engaging, the NLG may have been held in the down position by the overcentre mechanism of the drag brace assembly. Had this occurred, then external dynamic loads would be able to collapse the NLG.