On arrival at Crocker Island, during the prelanding checks, the left main landing gear would not lock down. The pilot diverted to Darwin where emergency services were available. He performed the emergency landing gear extension drill but the left main landing gear was still not indicating down and locked. The pilot flew past the tower for a visual inspection of the landing gear by the tower controller who advised that the landing gear "appeared normal". When emergency services were in place, the pilot landed. As the aircraft slowed on the landing roll, the left main gear collapsed. Subsequently engineers discovered that the left side brace centre bushing had seized on its bolt thereby preventing the left main landing gear leg from locking down. The operator's chief engineer has formally advised the Civil Aviation Safety Authority of the mechanical fault.