The aircraft was engaged on a multi-sector flight for the purpose of transporting bank documents. After the pilot completed normal pre-landing checks the aircraft made an apparently normal touchdown on runway 05. During the subsequent landing roll the undercarriage commenced to retract, resulting in the left main and nose gear legs collapsing. The aircraft skidded for a short distance before coming to rest 10 metres beyond the runway edge. No evidence was found of a pre-existing fault in the undercarriage system. It was considered probable the gear was inadvertently selected up whilst the pilot was attempting to raise the flaps during the landing ground roll.