Mr Horsley also admitted a charge of misconduct for knowingly making a false statement to the investigating New Zealand Law Society standards committee.
He had signed a statement saying he had not had a personal relationship with the client when he knew that to be false.
The tribunal suspended Mr Horsley from practice for a period of three years on that charge, to run concurrently with the suspension on the first charge. The suspension runs from April 4, 2014.
New Zealand Law Society's National Prosecution manager Mark Treleaven said three years was the maximum period of suspension from practice which could be imposed on a lawyer.
"The tribunal found that the acceptance of responsibility by Mr Horsley and mitigating factors were just sufficient to pull the penalty back from strike-off to the maximum period of suspension," he said.
"It is totally unacceptable for lawyers to enter into intimate personal relationships with a client where this would compromise the independence of the lawyer. Misleading or providing false information to a Law Society investigation is also viewed extremely seriously."
