VH-KDB commenced descent to Whyalla at 55km and advised Adelaide Flight Service (FIS) and broadcasted on local MTAF. Adelaide FIS advised "no IFR traffic" and no response was heard on MTAF. At 30km Whyalla, the estimated arrival time and intentions at Whyalla were broadcast and the pilots continued to monitor both radio frequencies (MTAF and Area). At about 10km from Whyalla, when leaving 3000 feet on descent, opposite direction traffic was seen on climb straight ahead. Rate of descent was increased as an evasive action and the other aircraft passed overhead. After passing, attempts were made to establish communications with the other aircraft on both MTAF and Area frequencies without success. A short time later, VH-LQD was heard on Area frequency and it was established that this was the aircraft seen. The pilot of VH-LQD, a foreign national, was flying a solo navigation exercise and although he had heard some transmissions, he had not associated them with conficting traffic.