A new threat has emerged for contract workers digging up communication pits owned by Telstra, with the pits found to possibly contain the deadly banned pesticide dieldrin.

The Communications Workers Union in Victoria has raised concerns workers could have been exposed to dieldrin that Telstra sprayed on its cables to stop termites.

The dieldrin link was flagged after recent reports about the disturbance of asbestos in the pits caused widespread concern about the rollout of the National Broadband Network (NBN).

Dieldrin is banned in most countries and was phased out in Australia in the early 1990s.

After once being widely used in agriculture and timber products, the toxic chemical has been linked with Parkinson’s disease and can trigger comas.

The union says it raised concerns about both asbestos and dieldrin with NBN Co at a meeting in August 2010.

A spokeswoman for Telstra confirmed dieldrin had been used but says it had stopped using the chemical well before the 1990s.

“We do not believe it is an issue however we will seek advice from chemical experts and relevant government authorities on the issue,” Telstra later said in a statement.

Experts say dieldrin exposure in telecommunications pits is unlikely to be as harmful as asbestos but they still urge action.

Professor Malcolm Sim said there could still be a concern about insecticides in the soil.

“They’re very persistent and as I said they can accumulate in the body and that was the main reason they were phased out,” he said.

A spokesman for NBN Co referred the ABC back to Telstra and was trying to track down more information.