The fob seal was taken from its exhibition case at Old Government House in Parramatta on Thursday.

The seal was on loan to the National Trust and belonged to Oxley, who was the Surveyor-General of NSW from 1812. The seal bears his family crest and is engraved with the motto "Patientia et Fortitudine" or "Patience and Strength".

Rebecca Pinchin, Collection Manager for the National Trust in NSW, said the seal is irreplaceable.

"It's a personal item that belonged to John Oxley, the explorer, it was made around 1800 so it's over 200 years old," she told AAP.

The seal is made of engraved rock crystal and mounted in an 18-carat gold converted fob-watch, and is about the size of a large ring.

Ms Pinchin said it was used by Oxley to "impress and seal correspondence".

The historian told AAP that staff and volunteers were very distressed by the disappearance of the seal.

"We're very hopeful police will be able to make some progress with their investigation," she said.

NSW Police said the theft occurred between 2.30pm and 2.45pm on Thursday and issued a description of a man who may know more about the incident.

He is described as having a dark complexion, about 175cm tall and of medium build and was wearing a black beanie, a dark top with a white stripe and carrying a small black backpack.

Australian Associated Press