A man accused of having sex with his 13-year-old niece yesterday walked free due to a legal blunder -- despite the judge admitting the evidence weighed against him.

The judge found yesterday that the error -- the prosecutor failed to have the girl's mother state her child's exact date of birth from the witness box -- was "fatal'' to the case and directed the jury to find him not guilty.

And Puisne Judge Norma Wade Miller told the 35-year-old defendant: "You are quite a lucky young man. Certainly in my opinion there was considerable evidence weighing against you. Watch your step.'' Earlier Mrs. Justice Wade Miller ruled that it was not enough that the girl stated her own date of birth, and her birth certificate backed that up.

The man -- who cannot be named for legal reasons -- was facing a charge of having unlawful carnal knowledge of a girl under 14 and three counts of sexual exploitation of a young person between August, 1995 and January, 1996.

His niece told the court how her uncle had claimed he was only 16 before kissing and fondling her, and months later having sex many times.

The judge told the jury that she had "agonised'' over whether to use her discretion to allow the error to be fixed.

But she decided against it because it could cause the public to lose faith in the justice system since the girl's age was "essential evidence''.

Coalition for the Protection of Children spokeswoman Sheelagh Cooper said the schoolgirl and her mother were both "distraught'' and hoped to appeal the decision.

Mrs. Cooper noted the alleged victim had already been put through the trauma of giving evidence in two separate trials.

She said the mistake lay with the Attorney General's office for putting an "unseasoned, ill-equipped prosecutor'' on such an important case.

The judge said that when a witness stated her own birthdate it was technically hearsay since it was repeating what someone else had said.

The submission for the case to be thrown out of court on the point of law was made by defence counsel Elizabeth Christopher before a single witness had given a word of evidence for the defendant.

Mrs. Justice Wade Miller said to allow the prosecution to reopen their case so the mother could simply state her daughter's birth-date would "bring the administration of the law into disrepute''.

Crown Counsel Sandra Bacchus admitted to the court she had not been aware of the legal hitch and argued the judge should let her reopen her case "for a second kick at the can'' since neither side had disputed the girl's age.

The case was a retrial after a Supreme Court jury deliberated for four hours in March but failed to reach a verdict.