A judge who ordered a defendant to read out insulting comments he had made about him on Facebook during sentencing has been given a rap on the knuckles by the High Court of New Zealand.

Troy La Rue had called Judge Allan Roberts a "f****** c***" with a "saggy chin" on Facebook - hours before he found himself in the New Plymouth District Court over unpaid motoring fines worth more than $6000.

He was sentenced to 300 hours' community service - but this was later cut on appeal because of questions about the way Judge Roberts reached his sentence.

The appeal decision has been praised by Megan Boyd, La Rue's solicitor, but condemned by the Sensible Sentencing Trust.

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At the original sentencing hearing, which took place in January, Judge Roberts asked La Rue: "Who are you talking about when you talk about, 'The f****** old c*** with the saggy old chin'?"

"Well, I guess I'm talking to you, sir," La Rue replied, according to a full transcript of their exchange. "And I, I don't really know what to say about that but I do apologise."

The details of their encounter were picked up by news outlets throughout the world - but the High Court was not laughing when the matter came before it on appeal last week.

"However amusing these exchanges may have been for the interested bystander, it is an inappropriate way to deal with a matter of unpaid fines," said Justice Robert Dobson, who heard La Rue's appeal.

"Predictably, counsel argued on the appeal that the judge set the number of hours community work, taking into account the irrelevant consideration of Mr La Rue's derogatory comments about him."

The appeal noted that the registrar's report to the judge had endorsed "250?" hours of community work in the judge's handwriting.

On the printed form, on which the judge records the outcome, the figure of 275 featured - but according to the appeal report this was crossed out and replaced with the figure of 300.

Megan Boyd, who represented La Rue, argued that either 250 or 275 hours were appropriate, but the 300 figure was decided while Judge Roberts, who has since retired, was dealing with the matter.

Justice Dobson conceded this was a "reasonable possibility".

"It therefore adds weight to the criticism that the judge was diverted from a relevant analysis of the appropriate substitute sentence by Mr La Rue's Facebook criticism of him.

"That concern is more than sufficient to justify revisiting what the appropriate sentence should be."

Justice Dobson decided that La Rue, who is on benefits and who had made no payments since his last community work sentence in May 2014, was subject to "some leniency" because of the "overbearing way in which the matter was dealt with in the district court".

He quashed the sentence of 300 hours community service and replaced it with an order for La Rue to complete 200 hours.

La Rue, who has no disposable income and who was described by Judge Roberts as "bone idle", saw his outstanding fines of $6,244 dropped in return for the community service.

In the wake of the verdict, Megan Boyd she was "pleased" with the outcome.

"I think that when you put people in positions of power you are entitled to expect the best standards of behaviour from them and that things are not taken personally," she said.

But the appeal court's decision was criticised by the Sensible Sentencing Trust, which said it sent out a signal to offenders that they could say what they liked without consequence.

"We need more judges like the judge who gave the original sentence," trust spokesman Garth McVicar said.

"No wonder New Zealand has such a high level of offending."

McVicar said the sentence rewarded an offender for making a mockery of the law.

"The judge is the one that's ended up getting smacked on the knuckles."