Margaret Beatrice McIntosh, whose late husband was former CSIRO chief executive Sir Malcolm McIntosh, sued the society for damages after suffering numerous injuries from the fall in March 2018.

At the time, Lady McIntosh was walking on a temporary stage erected for a recital of the St Matthew Passion on the altar of St Christopher's Cathedral Parish, an ACT Magistrates Court judgment published on Tuesday states.

She walked across the temporary stage, which was on top of the altar at the cathedral and comprised of risers that had been erected, before falling into a hazardous gutter gap.

At the time, Lady McIntosh, 71, was the newly-elected president of the society and one of the chorists rehearsing for the performance the next evening.

She sustained a fracture to the left wrist requiring surgery, a fracture to the left kneecap, permanent aggravation to the left shoulder, a cut to the left ankle and bruising to both legs.

Despite the injuries, she returned to St Christopher's to perform the next evening.

During a court hearing in early April, she said she was not warned about the gaps and that when she followed the route onto the stage set out for her, she was absolutely taken by surprise in negotiating the hazards.

"I fell down the two steps onto the marble floor and crashed into the screen, which was behind the riser," she said.

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The small, not-for-profit society had disputed most of the issues, particularly causation and the amount of damages.

It said while it had a duty of care, it criticised the plaintiff for a lack of expert evidence related to that duty being breached.

The society also argued that Lady McIntosh contributed to the negligence by failing to do a number of things, including keeping a proper lookout, taking reasonable care for her own safety and making alternative suggestions to remove the hazard.

Magistrate James Stewart said Lady McIntosh had put "forward a strong case on breach of duty".

"There were no immediate handrails, no physical barriers and no human guides to help the choristers traverse the gutter gaps or stop them from doing so," he said.

Mr Stewart said the society could have filled the gaps with timber structures or ramps and the failure to do so was unreasonable.

In relation to the society's criticism about the plaintiff's lack of expert evidence, Mr Stewart said he found "that the multiplicity, depth and width of the gutter gaps represented such an obvious obstacle that expert evidence on the degree of risk caused by them was superfluous and unnecessary".

"The defendant's case on causation is weak and defeated by the plaintiff's strengths on the civil standard," he said.

Mr Stewart said the defendant's case about contributory negligence failed.

"Forgetting that a hazard is present or not seeing a known hazard is not the same as being negligent," he said.

"The defendant has not been able to prove more than inadvertence, inattention or misjudgement on the part of the plaintiff in the face of an extraordinarily dangerous hazard created by the defendant."

Mr Stewart ruled in favour of Lady McIntosh against the society in the sum of $260,621.

Of that amount, $120,00 was for general damages, $32,500 for future out-of-pocket expenses for her left shoulder, and $24,598 for past out-of-pocket expenses.

The remainder of the sum includes interests, ongoing medical care and past and future domestic services.

The law firm for the society said they did not wish to comment.

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