In Biggles in Borneo, Telapur was a sawmill in peninsula Malaya (now Malaysia). The sawmill, which belonged to a pair of timber merchant brothers, Fee Wong and Ah Wong, stood on the Limpur River which crossed the peninsula from West to East.
When Biggles freed Fee Wong from Cotabato prison, Fee Wong brought news that an important Japanese convoy of rubber and tin was scheduled to cross the Malaysan peninsula on the Limpur River. It would pass his sawmill and could be attacked. His brother and the workers at Telapur would be available to help.
When Biggles got to Telapur, the convoy, comprising a large number of river barges, had already reached Telapur and was moored there under heavy guard. Unable to sabotage the barges directly, Biggles opted to go upriver and, with the help of Ah Wong's timber working elephants, push teak logs into the river,hoping that the swift currents would turn them into battering rams. Unfortunately, the logs got caught by a fallen tree and created a massive logjam, damming up the river. Kayan, Ah Wong's foreman, freed the logjam at the cost of his life and released a massive flood of water which destroyed the convoy at Telapur and also brought down a pontoon bridge further downstream, drowning a Japanese troop column which was crossing the river at the time.
Telapur is a fictional location and Johns did not supply much geographical detail to helplocate it.