Authorities have seized a gun made of matches and charged an inmate over the find at Mobilong prison in South Australia.

Jail staff made the discovery after they noticed an inmate behaving suspiciously.

Authorities say the 13cm replica was made out of matchsticks and had been painted black.

They described it as highly realistic.

Correctional Services Minister Tom Koutsantonis said it could have appeared threatening if used against prison staff.

"There is no doubt that if confronted with such an object staff would have felt threatened," he said.

"Our fear is that it could have been used for an escape attempt or to threaten other inmates.

"The gun has a position for a rubber band so it can be moved to seem it's being cocked like a real gun, at its tip are the ends of two pencils which have a metal casing for a rubber or eraser which were removed so if pressed against someone's neck or body from behind would feel as if it was cold, hard metal."

Mr Koutsantonis said the inmate had been moved from Mobilong prison, near Murray Bridge, to the high-security Yatala prison in Adelaide.

He said the prisoner had been banned from any type of craft activity.

Charges have been laid under the Correctional Services Act.