CONTAINER Terminal 10 on Lantau Island will be needed six months before its planned mid-1997 completion, says Marine Department director Tony Miller. The need would be prompted by the rapid growth in throughput, up from six million containers in 1991 to eight million last year, he said. ''Although the Lantau Fixed Crossing may not be completed by the time Terminal 10 is built, it will not be a problem to divert barges bringing goods from southern China to Lantau island,'' he said yesterday. Until the bridge was completed, Terminal 10 was being considered by port planners as an extended form of mid-stream operation, he said. The Government has planned on throughput growth forecasts of about 14 per cent, but the China boom pushed that past 28 per cent last year.