A report was received from the crew of a RAAF HS748 concerning the sub-standard operation of a Trinidad aircraft in the circuit area at Whyalla. The HS748 crew called entering the Whyalla MTAF stating their intentions. There was no response to that call. They then overflew the airfield and commenced a descent on the dead side of the circuit to join crosswind for a landing on runway 17. Descending through 1300 feet they observed a Trinidad on a reciprocal track about 200 feet higher. Repeated calls for the Trinidad to identify itself received no response. Shortly afterwards the pilot of the Trinidad made a downwind call which incorrectly advised that the aircraft was making an approach for another runway. The HS748 crew stated that the pilot of the Trinidad did not appear to understand standard MTAF procedures.