The cabling, about 1cm in diameter and worth about $16,000, was being used as a telephone line by Taieri Gorge Railway.
Taieri Gorge chief executive Murray Bond said the company had its own radio and an alternative telephone system, so the loss of cabling would not affect operations.
A KiwiRail spokesman said the theft, believed to have happened before September 21, also had not affected its rail operations and the company was not planning to replace it.
The thieves had put themselves at great personal risk by trespassing on to lines and into a tunnel used regularly by trains.
The company worked actively to minimise theft from the rail network and apprehend offenders, he said.
"We have a strong relationship with the police and the Scrap Metal Association and arrangements are in place for scrap metal dealers to report to both ourselves and the police anyone who is selling rail materials."
Dealers were supportive and were quick to report any instances, he said.
