JUDGES' mistakes in instructing juries have caused more than a dozen retrials this year and some judges are increasingly making the same errors.

The state's highest court has said several County Court judges had recently given juries wrong directions about the rules of cross-examination in criminal cases.

Court of Appeal judge Geoffrey Nettle made the comments about County Court appeals as the court granted retrials for Christian Brother John Francis Coswello, who was jailed for two years and 10 months for sexually abusing a 12-year-old ward of the state in an orphanage in the 1970s, and former teacher Stephen Peter Morrow, jailed for six years for getting his 17-year-old student pregnant and paying for her to have an abortion in the 1990s.

In each case, the Court of Appeal said the trial judges' mistakes had led to a ''substantial miscarriage of justice''.

The two judgments come as an Age analysis of criminal appeals decided this calendar year found that all but one of the 14 cases where retrials were ordered featured County and Supreme Court judges' mistakes in jury directions.

In the remaining case, the judge had failed to discharge a jury after it had inadvertently learned of the accused's prior convictions.

Eight of the 14 cases in which retrials were ordered featured allegations of sexual abuse, while three were murder cases.

High-profile cases where erroneous jury directions were among reasons for granting a retrial include:

■Robert Donald William Farquharson, accused of killing his three sons by driving into a dam.

■Donna Fitchett, accused of killing her two sons.

■Peter Norris Dupas, accused of killing Mersina Halvagis.

Mistakes made by judges in various cases included failing to direct the jury about the proper use of certain evidence, misdirecting them on the accused's consciousness of guilt, and the legal consequences of an acquittal by reason of mental impairment. Other mistakes included failing to warn a jury about the impact of pre-trial publicity and failing to direct a jury on the elements of consent in a sex case.

In July, the Victorian Law Reform Commission called for a single law on jury directions, new practices to help juries and better training for judges and lawyers. It found that half of the 538 appeals against conviction lodged in the seven years to July 2007 claimed the trial judge misdirected the jury.

The commission's project was sparked by a report on overly complex jury directions, produced by a judicial and lawyers' group led by retired Supreme Court judge Geoffrey Eames.

Court of Appeal president, Justice Chris Maxwell, said in a recent speech that the legal group would be reconvened to begin implementing reforms.

In the Coswello and Morrow cases, defence lawyers did not put the accused's version of events to the alleged victim in cross-examination.

In criminal law, a rule of cross-examination ensures that witnesses are given the chance to explain if the opposing side intends to contradict them.

The Court of Appeal ruled that County Court judge Thomas Wodak, who sentenced Morrow, erroneously instructed the jury that the failure of cross-examination enabled them to reject Morrow's assertions.

The court said that in the Coswello case, the directions given by County Court judge Ross Howie ''created a real possibility of the jury reasoning by impermissible means to a conclusion of guilt''.