Police are being independently investigated after detectives botched a rape trial late last year.

The man appeared in the Auckland High Court in November on eight charges, instead of the 22 he was meant to be tried on.

Prosecutors had to apply to have 13 charges heard in a separate trial after it was discovered the man's alleged victims - five women and five men - hadn't been told he had been arrested.

This meant the victims had no chance to prepare for the trial.

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The length and cost of jury trials varies considerably, but the Ministry of Justice estimated the average cost of a jury trial was $15,480 in 2007. A spokesman said this week there was no more recent estimate.

The offending allegedly occurred in the late 1990s and the man was charged after advances in DNA technology linked him to the crimes.

In early November last year, an Auckland Central detective began contacting the complainants and learned they had not heard from police since the late 1990s. Only one of the victims knew the man had been arrested and charged.

The detective was the third person to handle the file. The first is on maternity leave and the second had left the police.

In court documents obtained by the Herald on Sunday, former Crown prosecutor Sue Gray said there appeared to be "a combination of miscommunication between police officers and a lack of understanding on their part about the requirements of the committal procedure".

The detective was under the impression the trial was ready to proceed and she only needed to call the complainants to ensure they attended.

Gray told the court the complainants "had not been accorded the treatment required by the Victims' Rights Act".

Auckland City District Police Commander Mike Clement confirmed the prosecution did not follow standard police practice.

"Those aspects of the prosecution that haven't proceeded as per standard practice have been referred to the Independent Police Conduct Authority and police professional standards for an employment investigation," he said.

A new trial date will be set when the man appears in court this month. He has been in jail for the past 15 years for similar offending.