A jawbone almost 800 years old found on a Sydney beach this week belongs to a skull that washed ashore on the same stretch of sand almost six years ago to the day.

Forensic testing has today confirmed the jawbone found on Sunday belongs to the same girl of either Asian or Pacific Islander origin, whose skull was found on the same beach on September 12, 2008.

The remains of the girl, believed to be aged between three and five years old, have been dated between 1220-1400 AD.

Police were called to Mona Vale Beach, on Sydney's northern beaches, at the weekend after a resident discovered the remains among kelp at 10am.

Local area crime manager Inspector Craig Wonders told the Manly Daily the timing of the discovery was uncanny.

He said testing indicated the skull had been in the water for only a short time, while the jawbone showed signs of longer exposure to water.

It has never been established why the remains have only washed ashore in recent years or where they came from.