Shaun Bloomfield was charged with raping and sexually assaulting a girl, while his co-accused, Luke Merryful was charged with raping the same girl.

Mr Bloomfield and Mr Merryfull have both pleaded not guilty to the charges against them.

Judge Mark Dean said the first trial miscarried and was turned over by the Court of Appeal, the second trial miscarried because of the COVID-19 pandemic, and this one miscarried for a third time because the jury breached clear directions given.

"There is no room for jurors to conduct independent investigation whatsoever and that's what happened in this case," he said.

"You will be discharged without verdict. This trial is miscarried."

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The court heard the judge was informed by his associate that the jury had a question regarding the skylight of the caravan where the alleged rape took place.

Judge Dean said a juror investigated the case independently and conducted an experiment into the moonlight at 4.30am on April 7, against the direction of the court.

"The juror plainly conducted an experiment at half past four in the morning," he said.

The judge said the experiment was conducted at the same time in the morning the alleged assaults occurred in April 2016, suggesting it was an experiment using the light at that time.

Judge Dean said it was the third time these two young men had to stand trial which was a burden to them, the complainant, their families, and the community.

"All of those people have been affected by what has taken place and that is highly regrettable," he said.

Several people in the court room shook their head when the mistrial was announced, including one juror.

"I can only extend my apologies to everyone involved in the case," Judge Dean said.

"Highly regrettable what's occurred."

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He said he made it explicitly clear to the jury they should only listen to the evidence presented in court and should not investigate the matter themselves outside of court because that was the role of police.

The court heard that it was a flagrant breach of the Juries Act 2000 and a clear breach of directions given, and it was the judge's responsibility to refer the matter to the juries commissioner, who will refer it police to deal with at their discretion.

"It is in the public interest that jurors abide by direction given," he said.

The judge said he "had no choice but to do this (declare a mistrial)".

The retrial will be moved from Bendigo to Melbourne to access better technology.

The retrial will be heard before Judge Dean on August 8 in the Melbourne County Court, and Mr Bloomfield and Mr Merryfull's bail is extended to that date.

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