The aircraft was scheduled to complete a flight from Brisbane to Townsville with intermediate stops at Blackall, Longreach, Winton, Richmond and Hughenden. The weather forecasts received by the pilot indicated that an alternate destination would be required for Blackall and Longreach. Charleville was suitable as an alternate, however, the terminal weather forecast required the carriage of an additional 30 minutes holding fuel for aircraft planning to land there and the pilot flight planned accordingly. While the aircraft was enroute to Blackall, the weather forecast for Charleville was amended to require the carriage of 60 minutes holding fuel. The amended forecast also rendered Charleville unsuitable as an alternate after 1130 hours. This forecast was not passed to the pilot by either of the two Flight Service Units (FSU) concerned with providing information to the aircraft. The pilot was thus unaware of the change in forecast weather at Charleville. However, prior to landing at Blackall at 0944 hours, the pilot requested and received actual weather reports from both Longreach and Charleville. These reports indicated that Longreach was still available as a destination and Charleville was still suitable as an alternate destination. After departing Blackall the aircraft proceeded to Longreach. The Automatic Terminal Information Service (ATIS) at that station gave the the cloud base as 400 feet and the pilot attempted two instrument approaches. Both of these approaches were unsuccessful and the aircraft was diverted to the alternate, Charleville. Reports of the actual weather at Charleville, which were passed to the pilot before his decision to divert, confirmed that Charleville was a suitable alternate destination. Some 32 minutes later, while enroute to Charleville, the pilot received a report from Charleville FSU that the present weather at that station was a cloud base of 1000 feet, with some lower patches and the visibility to the north was two kilometres in rain. When the aircraft arrived overhead Charleville at the published minima, the pilot reported that it was between cloud layers with about seven eighths of cloud below. He immediately commenced a diversion to Roma and gave the total fuel endurance as 60 minutes. He was then advised that the aircraft had been sighted from the ground as it flew overhead. Prior to his decision to divert, the pilot had been advised that the pilot of another Fokker F27 aircraft, which had just landed at Roma, reported that the weather conditions there were fluctuating about the minima. While the aircraft was enroute, attempts by Charleville FSU to find a more suitable landing point were unsuccessful. Arrangements were then made with an aircraft charter operator at Roma to advise the pilot on the method of approach most likely to achieve a landing. The aircraft subsequently landed at 1227 hours after being forced to descend below the published instrument approach minima. After landing the remaining fuel endurance of the aircraft was approximately 15 minutes. At 1244 hours, another F27 aircraft diverted to Brisbane from overhead Charleville after two missed instrument approaches.