Auckland Prison inmate Arthur Taylor does not have the permission of the prison director to earn an income while behind bars, and a Corrections spokesperson has confirmed the department will be looking into it.
The self-styled jailhouse "lawyer" told The Nation's Lisa Owen that he gets paid for what he calls consultancy work.
"I am contacted frequently... from people who seek advice on various matters and are only too willing to pay," he says.
In 2010, media reports claimed Inland Revenue had assessed his income for the previous year at more than $100,000.
He refuses to confirm or deny that, and won't say how much he has banked over the years.
"I pay my taxes. I declare all my income, I pay child support," he says.
Inmates who are self-employed and earn income while in prison need approval from the prison director for their work.