The Corrections Department is being accused of unreasonably and irrationally denying access to a prison and a judicial review could be sought in the High Court on the matter.
Roger Brooking, a drug and alcohol clinician, says he has worked from Arohata prison, Wellington, for the past decade, assessing inmates at their lawyer's request.
But he says the prison now considers him an unapproved visitor and has banned him without explanation.
Corrections denies he is banned and says it is working to process his visitor's application.
Lawyer Michael Bott says he will seek a judicial review in the High Court if the standoff is not resolved by early next week.
He says the matter is urgent, given upcoming parole hearings for prisoners.