He unsuccessfully tried to place Mr Marchant in a headlock and then grabbed his shoulder. Mr Marchant wriggled free and Wereta was restrained by security.
Back in the same court today, Wereta was jailed for a year by Judge Russell Collins, who said he had no alternative but to hand down a prison sentence.
It will be served on top of a lengthy term imposed on Wereta last year for aggravated robbery.
"I'm not going to give you a lecture. I'm not going to go on about what happened," the judge told Wereta.
"I thank you today for listening patiently to what I have to say."
Away from court, Mr Marchant told APNZ he wasn't in any pain because of the assault, but was annoyed with Wereta.
Nothing like that had happened to Mr Marchant before. He had received threats, but never anything physical.
As Wereta was leaving the dock Mr Marchant said he saw Wereta "charge" at him.
"I could see what was going to happen.
"I put my arm up, really thinking to myself I was going to be punched."
A Ministry of Justice spokesman has said of the incident: "The judge and the registrar immediately left the courtroom and remained in a secure locked place until the defendant had been removed from the court."
Meanwhile the Department of Corrections has said early indications from an "operational review" seemed to suggest the incident was "well managed" by security officers and court staff.
"Prisoners can be volatile and unpredictable and many have long histories of antisocial behaviour and resort to violence with little warning," Corrections northern regional commissioner Jeanette Burns said.
"The prisoner was being escorted by First Security staff contracted to the department at the time of this incident."