Helen Milner, dubbed the 'Black Widow', has been attacked while in prison.
She was punched by two inmates in Arohata Prison, near Wellington, Fairfax Media reports.
The prison's director Chris Burns said a woman had been assaulted but was not injured.
In 2013, Milner was convicted of fatally poisoning her second husband Phil Nisbet.
She is currently serving a life sentence with a minimum non-parole period of 17 years.