He told Fairfax Media the gel didn't stop water from flowing into the joints, the most environmentally vulnerable part of the network. It allowed it to penetrate up to 1.5 metres of cabling and accelerated the corrosion process.

He had seen the damage first hand as one of thousands of workers contracted by Telstra to eradicate the gel from joints in Darwin, north Western Australia and Queensland, he said. Those regions we more susceptible to the effects of decay because of the warmer and more humid climate.

"The majority of the places where it was installed were closest to the customer," Mr O’Donnell said. "This is important in the concept of fibre to the node. The highest activity levels for a communication technician when they are doing installation and fault rectification work are generally in that last mile."

Telstra spokesman Scott Whiffin declined to comment on the extent of the problem or whether the gel was accelerating copper's decay, but he said Telstra invested hundreds of millions of dollars a year to improve and expand its coverage.

"We haven’t released that information but we are constantly installing, replacing, repairing and upgrading copper lines throughout the network. And in the main, and for the vast majority of our customers, our copper network provides a high quality fault-free service," Mr Whiffin said.