The operators of Sydney's light rail say it could be several days before services are back up and running.

Two derailments in Rozelle Bay and Wentworth Park this morning forced all services to be temporarily suspended.

The company Trans-dev says an investigation is under way into the cause of the incidents, but it is believed a network technical fault is to blame.

Both vehicles were operating at reduced speed at the time of the derailments, as they approached stops, and no-one was injured as a result.

Transdev Deputy Managing Director, Matt Baynie, says it is the first such incident of its kind in 15 years of the operation.

"Transdev will be working with the relevant authorities to determine the cause of the two derailments," he said.

"Operational and customer safety are key network considerations for Transdev and as a result we cannot operate services until the fault is thoroughly identified, investigated, rectified, and tested to be safe. We expect this to take several days."

Buses will replace all light rail services until further notice and customers can also use their light rail tickets to access buses, CBD and Inner West Line trains and ferries at no extra cost.