When the aircraft arrived in the Broken Hill circuit area, the pilot observed the airfield covered in fog. After failing to become visual during an instrument approach, he decided to divert to Wilcannia. The pilot declared an emergency during the diversion, as he expected to arrive at Wilcannia with less than minimum fuel. The aircraft landed at Wilcannia with about 25 minutes of fuel remaining. Investigation showed that the Terminal Area Forecast (TAF) requested by the pilot prior to the flight did not include a forecast of fog or any other significant weather at Broken Hill. At the time of issue of the forecast (1850 Z), the conditions at Broken Hill were considered to be too dry for fog to develop. Satellite images did not show any fog on the ground. At 2125Z, after receiving advice of increased moisture levels at Broken Hill and reports of fog at Mildura, the TAF for Broken Hill was amended to include a 30% probability of fog. The incident aircraft arrived overhead Broken Hill aerodrome at 2128 UTC and began an instrument approach. Another company aircraft had landed at Broken Hill approximately 30 minutes earlier, and had advised the pilot of the incident aircraft that there was no fog. .