Federal Court Judge John Nicholas overturned that last year, rejecting the tribunal's findings that the sex had to be condoned by the government if she were to qualify for compensation.
"If the applicant had been injured while playing a game of cards in her motel room, she would be entitled to compensation even though it could not be said that her employer induced her to engage in such activity," Nicholas wrote then.
Comcare lost its appeal to the Full Bench last December, with the three judges finding that the government's views on the woman having sex were irrelevant.
But the High Court ruled that Comcare was not liable to pay compensation. The judges did not say how much compensation had already been paid. Comcare declined to comment on the amount, but said it was considering recovering it.
"The relevant question is: Did the employer induce or encourage the employee to engage in that activity?" a summary of the court ruling said. A majority of judges Justices Robert French, Kenneth Hayne, Susan Crennan and Susan Kiefel answered "No."
Justices Virginia Bell and Stephen Gageler dissented.
The crucial facts were that the overnight stay was within the two-day period of the work trip and her employer had encouraged the woman to stay in the motel in Nowra, 160 kilometers (100 miles) south of her hometown of Sydney, Gageler said.
"In the absence of any suggestion that she was engaged at the time of injury in misconduct, those facts were sufficient to conclude that the injury the respondent sustained during that interval, and when at that place, was sustained in the course of her employment," Gageler said.
Employment Minister Eric Abetz hailed the ruling as a victory for common sense.
"Instances such as this, where an employee seeks to stretch the boundaries of entitlements, are of great concern and the High Court's intervention is welcome," Abetz said.
Comcare declined to say what effect the ruling would have on other compensation claims. The government insurer also declined to say whether other claims in comparable circumstances were pending.
Australian National University law lecturer Cameron Roles said the ruling reduced the range of after-hours activities covered by workers' compensation when an employee was away from home on business.
"I don't think you can say that this decision means that all activity in hotel rooms won't be covered, but it certainly narrows that range of what activity will be covered," Roles said.
"In terms of the precise activities, it's quite difficult to say because everything turns on the facts" of an individual case, he said.