The tribunal in May said McKay's conduct was an abuse of his position as a lawyer and a serious breach of his duties to the woman.
McKay is appealing against the misconduct finding made against him.
"Mr McKay's failures to this client were many and obvious," Judge Clarkson said in the decision on penalty. "The consequences to her of his failures were devastating. He enabled her home to be purchased by another client at a gross undervalue."
According to the tribunal decision, McKay's defence involved an attack on the honesty and integrity of his client.
"We considered Mr McKay attempted sophistry and at times plain untruthfulness to escape facing up to the responsibility of his acting in this position of irremediable conflict," it said.
The decision said McKay's submissions on what penalty was appropriate "lacked any element of remorse or insight into his failures".
"They were self-pitying and relentlessly self-absorbed."
Although McKay said he wanted a second chance, the tribunal considered it necessary to strike him off.
"We can have no confidence that Mr McKay would not, if able to continue to practise, repeat his actions, given his sense of self-righteousness and justification for his actions," Judge Clarkson said.
McKay was ordered to contribute $70,000 to costs and reimburse the Law Society $14,812 for hearing costs.
Law Society president Chris Moore said McKay had "strayed so far from the path of professionalism that he can justifiably be said to have ceased to function meaningfully as a lawyer at all. It is rightly expected lawyers act for their clients without conflict of interest or any form of bias."
