The aircraft was inbound to Adelaide from Ceduna and had been cleared by ATC to descend from 5000 feet to maintain 1000 feet AMSL. The aircraft was observed on radar to be passing 2600 feet on descent at about 46km north west Adelaide but shortly later the return had disappeared. A Distress Phase was initiated when communication could not be re-established with the aircraft. Another aircraft in the area reported a cloud base of about 600 feet with rain showers. Later interpretation of the radar tape shows the aircraft levelling at 1000 feet then commencing a steepening descending left turn until lost from the radar at about 300 feet.