Justice Warwick Gendall sided with Ms P whose appeal was heard before him earlier this month.
"This case in the Family Court involves what are generally difficult and sensitive allegations of domestic violence. Factual matters and issues of credibility are critical in hearings of this kind. Impressions of impartiality are particularly important," he said.
"There is a real possibility that, in the eyes of a fair-minded and fully informed observer, the Judge might not be seen to be impartial in reaching a decision in this case."
Ms P, a "litigation lawyer" practising in Dunedin, said she had previously appeared before Judge A in a professional capacity in both the District and Family Court.
And the link ran deeper.
Her daughter was friends with Judge A's daughter Ms P said she knew the judge's wife from pick-ups and drop-offs at school.
The pair had "regular coffee dates" with other mothers, the court heard, and the lawyer had shared intimate details of her break-up with Mr Q.
"Judge A noted that he had no knowledge of any conversations Ms P had had with his wife. If they had discussed any sensitive information, he said that his wife would have never passed it on to him," the judgement said.
"He considered that Ms P was not in a different category to anyone else who appeared before him."
Defence counsel Len Andersen said it was "concerning" the judge appeared to have already made adverse findings against his client before the hearing had begun.
While Justice Gendall said the issue was "finely balanced", he upheld the appeal.
"Unlike the conclusion reached by Judge A as to his previous interactions with Ms P, I am of the view that these interactions, coupled with her past professional contacts with him as a lawyer, place her in a different category than others who appear before him as litigants in this difficult and highly charged area of alleged domestic violence."