Chief Justice Lucy McCallum was forced to abort the ACT Supreme Court trial of former Liberal Party staffer Bruce Lehrmann on Thursday, when she said a sheriff's officer had spotted an academic paper about sexual assault in the jury room.

The judge said the officer had accidentally bumped a juror's document folder onto the floor during "routine tidying" of the room on Wednesday afternoon.

When he picked up the clear folder to put it back on the chair from which it had fallen, he noticed it contained the research paper, which was not in evidence.

The sheriff's officer subsequently took steps to alert Chief Justice McCallum.

Chief Justice McCallum had warned jurors at least 17 times not to undertake their own research during the trial of Lehrmann, who has pleaded not guilty to a charge of engaging in sexual intercourse without consent.

Lehrmann, 27, denies any sexual activity with former colleague Brittany Higgins in March 2019, when he is alleged to have raped her on a couch at Parliament House.

The jury had been deliberating for a week when the rogue juror's misconduct was discovered, prompting Chief Justice McCallum to discharge all 12 members on Thursday morning without a verdict being reached.

"At the very least, the fact that the paper was located and taken into the jury room by the juror indicates that it may have influenced that juror's contribution to the jury's deliberations," the judge said.

"The unfairness to both [Lehrmann and the prosecution] is manifest."

Chief Justice McCallum later published written reasons for her decision to declare a mistrial, revealing in a footnote that more problematic material had been found.

"Following the discharge of the jury, I was informed by the sheriff's officers that the same juror was also in possession of two additional academic articles on the topic of sexual assault," the territory's top judge wrote.

In NSW, the juror's misconduct would constitute a criminal offence with a maximum penalty of two years in jail, a $5500 fine, or both.

"There is no such offence in the Australian Capital Territory," Chief Justice McCallum said.

Before adjourning on Thursday, the judge urged media to "fall silent" on the case after reporting on the mistrial, noting she had already listed Lehrmann's potential retrial to commence next February.

She also expressed her gratitude for the "exemplary" work of counsel for both the prosecution and defence during the aborted trial.

"The role of counsel in criminal trials, particularly defence counsel, is sometimes poorly understood," Chief Justice McCallum said.

"In this trial, all counsel have conducted themselves with the utmost integrity, fairness, honesty and, perhaps most importantly, fearlessness.

"They are to be commended, not criticised, for doing so."

MORE COVERAGE OF THE TRIAL: