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The suicide of a prisoner at Christchurch Men's Prison is being investigated as his family plans to sue the Department of Corrections.

Michael Graham Maxwell was taken off life support on October 16, six days after being found in his cell.

He was serving a sentence for assault, kidnapping and sex with a minor.

Maxwell had been seeking treatment after being raped by cell mate Dean Noble in 2005, The Press reported.

A parole report from January 2009, while Maxwell was being held at Wanganui Prison, backed his request.

"He needs to access ACC counselling for personal issues. It is apparently not available at this prison. It is an urgent need for him and we strongly support and recommend it," the report said.

Maxwell wrote to the Corrections chief executive and the Prime Minister asking for help and to be transferred back to the South Island.

A corrections manager responded saying he would be transferred to child sex-offenders programme at Rolleston.

There he complained of another prisoner looking at him "in a sexual way", his lawyer Shannon-Leigh Litt said, and was transferred back to Christchurch Prison following drug and violence offences.

A prison source told the newspaper Maxwell killed himself after prisoners began taunting him, saying Noble was returning to the prison.

Maxwell's sister Tania said the rape had changed him.

"Even on [home detention] we could see what it had done to him. It made him into a very angry person."

She said at the time of his death he had been on suicide watch.

"He was not looked after."

Corrections said it could not comment as the death was subject to a review by the independent Prison Inspectorate.