On August 2, George Charlie Baker was found guilty by an Auckland District Court jury of kidnapping, threatening to kill and assault with an offensive weapon.
The charges were laid after Baker took an elderly prisoner hostage with a weapon and attempted to kidnap a prison officer in Auckland's Paremoremo Prison in August last year.
Baker committed the crime while serving an 18-year sentence for the murder of Liam Ashley, who was killed in the back of a prison van almost four years ago.
He allegedly sent a letter to the juror midway through his trial after getting her contact details from a list of jurors' names, addresses and occupations.
Baker was entitled to access the list under supervision because he represented himself at the trial.
Mr Key said the reports Baker had contacted a juror were disturbing and changes may need to be made.
"It seems unacceptable that someone that's before the courts would be able to contact a juror in that way," he told Newstalk ZB this morning.
Justice Minister Simon Power was expected to meet ministry officials today to discuss the situation.
It is understood Baker's letter to the middle-aged married woman was not intimidating, but contained romantic overtures.
Ministry of Justice deputy courts secretary Andrew Hampton said he was taking the matter very seriously.
"This hasn't happened before, certainly in my time in the ministry. Jurors need to be assured that they won't be subject to even a hint of intimidation so that's why we're taking it very seriously."
