The wife of New Zealand anti-whaling activist Peter Bethune says he is being held in virtual isolation in Japan with rapists and murderers.
Mr Bethune was detained after boarding a Japanese whaling vessel in the Southern Ocean in February.
He is in prison in Tokyo where five charges have been laid against him - with the most serious carrying a maximum jail term of 15 years.
His wife, Sharyn Bethune, told Morning Report that she has received three letters from him and it appears he is being held in maximum security and kept in his cell about 22 hours a day.
Mrs Bethune says it appears he is being used as a political target.