During a two-day hearing last month, the tribunal heard from a QC with more than 30 years' experience in criminal law.
The lawyer said that even at Mr Hart's "very high" hourly rate of $1000, his work on a case for one family could have been done for between $15,000 and $16,000.
The tribunal, chaired by Judge Dale Clarkson, found Mr Hart's fee was between 95 per cent and 107 per cent above a "reasonable fee".
"We consider this to be gross overcharging."
It said Mr Hart could not justify the time spent on some of the court appearances and preparation. It also said he had not told his client's family about his fee structure.
Another charge of failing to inform a private investigator that his bill would be paid only if he got legal aid was also proven.
The tribunal heard evidence about the investigator's fees of $4682 going unpaid until a complaint was lodged with the Law Society.
Mr Hart then paid half the fees before paying the rest only after the matter was referred to the Disputes Tribunal.
A third charge centred around Mr Hart's refusal to disclose a file to the Law Society's Standards Committee after a complaint over costs.
Despite a number of extensions for time, Mr Hart continued to delay handing the file over for more than a year.
The tribunal found Mr Hart had prevented the Standards Committee investigating the complaint.
"We regard this as an extremely serious breach of professional standards and most certainly reaches the level of professional misconduct."
The tribunal has called on the Law Society and Mr Hart to make submissions regarding a penalty.
Both parties have two weeks to file their arguments.
Mr Hart's hearing took place last month at the Auckland District Court but notably Mr Hart was not present.
The hearing had been adjourned five times when Mr Hart's lawyer, Nigel Cooke, asked for another on account of Mr Hart suffering from chest pains, breathing difficulties and fatigue.
In his closing submissions, Standards Committee lawyer Paul Collins told the tribunal that he had argued a one-sided case but said the evidence was there for the tribunal to find Mr Hart guilty of misconduct.
"The tribunal is entitled to take from all it has seen from Mr Hart's conduct in this proceeding itself to draw from that a constant theme ... which is one of contempt to his professional responsibilities," Mr Collins said.
Mr Hart told APNZ that he would be appealing the tribunal's decision to the High Court at Auckland but would not comment further.
