Brocklands nursery at Winkleigh sued TasNetworks for $2.5 million in damages, claiming a circuit breaker at Glengarry cut off power and switched it back on again four times in quick succession, causing high voltage electricity to enter their lower voltage system and destroy a potting machine.

The incident occurred after a tree fell on powerlines, and Brocklands claimed that hardware on a pole had been installed incorrectly by Aurora Energy, which has since transferred its assets to TasNetworks.

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The grounds for appeal included negligence, circumstantial evidence as to the cause of the power surge and that the judge made errors in weighing up this evidence.

The full bench of the Supreme Court found that the pole was "poorly designed and installed" and did not comply with standards, that spark gaps were misaligned, the arc gap rods should not have been painted, the earthing system was not sufficient and that "catastrophic damage" had been caused to the potting machine as a result.

The specific reason for the pole's malfunction was contested among the electrical experts who gave evidence, but no other reason was found for the destruction of the potting machine, meaning the fault must have been caused by the pole's equipment.

The fault has not reoccurred since insulation was installed on the pole, which the court considered to be "an extremely significant piece of circumstantial evidence" demonstrating that the pole had been at fault in 2010.

The full bench of the Supreme Court found that the trial judge placed too much emphasis on the evidence of some experts over others.

Chief Justice Alan Blow said it was "hardly likely" that something went wrong at the Brocklands property to cause the damage.

"On that basis I am satisfied on the balance of probabilities that the damage was caused by one or more breaches of duty for which the respondent is responsible, but I am not satisfied on the balance of probabilities that the damage was caused by any particular breach of duty," he wrote.

"Breaches of duty have been established in relation to the installation of the pole without an adequate separation between the right hand bushing and the "a" conductor, and with inappropriate spark gap separations, and in the failure to rectify the first of those defects over many years."

Justice Robert Pearce said Aurora Energy had breached its duty of care.

"Whichever of the three possible mechanisms, or whichever combination of them, caused the surge, I am satisfied that it was as result of a breach of the respondent's duty to take reasonable care to avoid foreseeable harm to the appellant," he wrote.

An assessment of damages will be carried out by another judge.