The lead witness in a gangland murder trial has been bashed after prison guards placed him in the same cell as the accused killer he was testifying against.

Non-association orders were sent to Corrective Services NSW, however, sources told The Daily Telegraph the witness and the accused were put in the same area of Silverwater Metropolitan and Remand Centre on Tuesday.

The injuries sustained by the witness meant he was unable to face Wollongong Local Court on Wednesday where he was due to give evidence in the trial of the Victorian man.

The lead witness in a gangland murder trial has been bashed after prison guards placed him in the same cell as the accused killer he was testifying against (stock image)

A spokesman for Corrective Services NSW has confirmed an inmate was treated for 'minor injuries' and, while it is unclear who carried out the assault, an investigation is underway.

'This was listed for a committal hearing specifically for a witness to give evidence… that witness is not in a fit state to give evidence today or for the next few days,' Crown prosecutor David Scully told the court on Wednesday.

An explanation has been sought from Minister for Corrections David Elliott and 'reassurance that it will not be repeated.'