NOXIOUS gases coming from sewage and industrial waste being pumped into Kai Tak nullah are corroding the new helicopters of the Government Flying Service, putting safety at risk and costing hundreds of thousands of dollars a year. The service's pilots and staff working within metres of the nullah have complained of irritation to the eyes and of dreadful working conditions caused by the foul smell. Sensitive electronic and computer equipment is corroding four times faster than engineers expect and the gases are even eating into unreactive metals such as gold and silver. Silver-plated electronic plugs on communication equipment of the eight S76 helicopters turn black within days, according to the service's engineers. Navigation equipment is constantly being sent overseas to be repaired with the solid gold connection pins turning black despite being virtually sealed within consoles. The paintwork of all the service's aircraft, including the Super Kingair fixing planes, is constantly treated for corrosion, taking up vital man-hours. Service controller Brian Cluer has lobbied the Environmental Protection Department during the past nine months without success. He and others within the organisation are concerned about the affects of the gases on the aircraft which provide vital search and rescue operations and casualty evacuations. ''If it is doing all this to the aircraft then what must it be doing to us,'' he said. ''Many of my pilots have complained about problems with their eyes. The smell is absolutely terrible, especially in the summer. It is a dreadful working environment.'' Sewage and industrial effluents are discharged into Kai Tak nullah and Kwun Tong typhoon shelter from nearby domestic and industrial areas. A variety of noxious and corrosive gases are believed to be coming from the nullah including hydrogen sulphide which has a characteristic ''rotten egg'' smell. The long-term effects on humans even in small concentrations include eye irritations, fatigue, sleepiness, headaches, nausea, poor memory, anxiety and dizziness. At higher concentrations it can lead to respiratory problems and convulsions. The Black Hawk has a computerised automatic hovering system which allows the pilot to engage in other matters while the aircraft is flying. Chief engineer Graeme McIntosh said that if it was affected by corrosion and failed there was potentially a very serious problem. ''I suppose the worst scenario is that it could cause a safety problem,'' he said. Principal Environmental Protection Officer Dick Rootham said hydrogen sulphide was probably one of the major gases coming from the nullah. But he said a $1.3 billion programme of sewerage replacement was under way to direct the waste further out into Victoria Harbour. However, it will not be until 1997 at the earliest that a noticeable difference will be achieved as far as the smell is concerned.