An average of 160 truck cycles are occurring daily between the Mayfield site and Kooragang Island.

A BHP spokeswoman said two types of dredged sediment were being transported.

The first sediment consists of immobilised or treated material that does not meet the NSW general solid waste classification.

The main contaminants in this sediment are polycyclic aromatic hydrocarbons, in particular naphthalene, produced from coal tar contamination in the old coke ovens.

Other toxins include ammonia, cyanide and benzene.

"Individual contaminants within the matrix of the sediment will obviously represent different levels of toxicity," the spokeswoman said.

"For example, [the hydrocarbons] are primary contaminants that vary in their type and concentration, whilst benzene is recorded at typically very low levels."

The material is being buried in a purpose-built landfill designed to prevent interaction with ground and surface water.

About 80,000 cubic metres of this material has been moved to date.

The second type of sediment complies with the general solid waste classification. About 210,000 cubic metres of this material has been taken to a Port Waratah Coal Services building designed for dredged fine materials.

The spokeswoman said stringent safety measures ensured safe transportation.

These included random speed and transport route checks, daily drug and alcohol testing of drivers as well as the covering of truck loads and washing of trucks before they leave the site.

"All sediment material transported off-site has been confirmed from testing as not constituting a hazardous substance or dangerous good," the spokeswoman said.