Forensic teams are examining human body parts found on the F3 freeway on the New South Wales central coast last night.

A passing motorist called police around 11:45pm (AEDT) last night after spotting the remains on the freeway near the Gosford off-ramp at Somersby.

Police blocked off two north-bound lanes between Mooney Mooney Creek Bridge and the Kariong interchange and set up a crime scene.

They are now trying to identify the remains and work out how and when the person died.

Police are also investigating how the remains got onto a stretch of road that is not accessible to pedestrians.

David Wright, from the Roads and Traffic Authority's transport management centre, says the discovery is causing major congestion.

"We're looking at around 4 kilometre queues prior to the actual lane closures for the north-bound run, so for that reason motorists are advised to avoid all non-essential travel," Mr Wright said.

Police are calling for drivers who saw anything suspicious on the freeway last night to contact them.

Anyone with information can call Crime Stoppers on 1800 333 000.