AN investigation is underway after a tunnel was found under Goulburn maximum security jail.

Corrective Services NSW has confirmed a “man-made cavity” was discovered under a cabinet in the workshop area of the prison.

A spokesman told news.com.au the cavity was 100m away from the prison walls — and conceded they didn’t know who made it or what its “intended use” was.

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“The cavity is about 600 millimetres deep and its entrance is 400 millimetres by 180 millimetres,” the spokesman said.

This morning 2GB is reporting a murderer serving a long jail term has been accused of making the tunnel.

The workshop remains closed to inmates while the investigation is underway.