A judge has taken the unusual step of overturning a man's conviction for rape in a harsh criticism of the advice given by his defence lawyer.

The man pleaded guilty to three sex charges, including one of rape, on the morning the jury trial was scheduled to start in August 2016.

But in a stinging ruling obtained by the Herald on Sunday, Judge David McNaughton said there had been a "substantial" miscarriage of justice in the case.

He believed the defendant was pressured into pleading guilty because his lawyer, Arlan Arman, was not prepared for the trial and did not give "competent or correct" legal advice.

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"While Mr Arman claimed to be one of the best lawyers in Auckland on the basis of service to clients and results achieved, what emerges from the evidence here is the complete opposite," wrote Judge McNaughton.

The district court judge allowed the man to "vacate" the guilty pleas he entered to the serious charges. A new trial will be held.

In response to Herald on Sunday inquiries, Arman said he "strongly" disagreed with the ruling of Judge McNaughton but could not challenge the findings.

"The Judge has chosen to accept a version of events which was presented to the court by a person who had pleaded guilty to sexual assault and sought to have that conviction set aside," Arman said in a statement.

"I was a witness in this case, not a party to the proceedings, and have no legal standing to challenge the court's decision. I strongly disagree with the judgment, but have no recourse to challenge it."

The 33-page judgment was issued in December after an application by the man's new defence lawyer, Shane Tait.

He could not comment ahead of his client's new trial, except to say he was considering a complaint to the Law Society.

According to the defendant's affidavit, he felt pressured to plead guilty at the last minute when Arlan Arman said he was unlikely to be acquitted at trial.

In his evidence, the man said Arman had not thoroughly discussed the evidence against him and he was never given a copy of the police file.

Arman disputed that, saying there had been numerous and lengthy discussions about the evidence, but acknowledged he did not provide a copy of the police file because the client did not ask for it.

The file was later lost.

And in the 12 months leading up to the trial, Arman had instructed an "agent" lawyer to act on his behalf at court appearances instead of turning up to court himself.

In analysing the case, Judge McNaughton said there was always an "arguable defence" to the charges and identified evidence of a potential motive for a false complaint of rape to be made.

He criticised Arman for failing to make enquiries into that potential motive, or interview potential witnesses, as well as failing to seek information from other government agencies.

Even without those enquiries and the possible motive for fabricated allegations, Judge McNaughton said the case was "eminently defendable".

"In the absence of corroboration, it was her word against his."

Judge McNaughton described the man who pleaded guilty to the sex offences - and his partner who corroborated parts of his evidence - as "reasonably earnest and straightforward witnesses".

In contrast, Arman - who advertises himself as one of the best lawyers in Auckland - was "ultimately a completely unconvincing witness".

In Judge McNaughton's ruling, he said Arman was not properly prepared for the trial on August 2016.

"The inevitable inference is that Mr Arman quickly lost interest in this file after receiving no further payments past the case review stage."

He said the lawyer should have withdrawn from the case at a much earlier stage to protect himself and in the best interests of his client.

"The only conclusion I can come to is, that at the last moment, with a firm fixture imminent and no prospect of further adjournments, Mr Arman pressured his client into pleading guilty by giving him misleading advice," said Judge McNaughton.

"The applicant did not receive competent or correct legal advice. He did not plead guilty on an informed basis, and he certainly did not do so freely.

"In this case, there has been a substantial miscarriage of justice."