SafeWork inspectors are investigating a Lower Hunter Valley worksite where a man was killed by a falling portable building.

Authorities were called to the Newcastle branch of the equipment rental business Onsite Rental Group on Huntingdale Drive in Thorton, NSW on Wednesday morning. 

"Initial inquiries indicate the worker was unloading a portable amenities block from a vehicle when he was struck by the load and suffered crush injuries," a SafeWork NSW spokesperson said.

The man in his mid-50s were pronounced dead at the scene.  He is believed to have lived in the Bonnells Bay area in the Lake Macquarie region.

"Our thoughts and sympathies are with the family, friends and colleagues of the deceased.  The precise details of the circumstances leading to this tragic incident are not yet known," said Onsite chief executive Mike Foureur, as quoted by Fairfax Media.  "Work health and safety at our sites is our number one priority.  We will assist and cooperate with all relevant authorities in their investigations and are doing everything possible to support the family and other employees affected by this tragedy."