This included McCarthy being the accountant for companies and trusts but also having a personal interest in other firms linked to this client.
When deciding on a penalty, tribunal chairman Mike Whale said McCarthy had acknowledged he had a responsibility to ensure the agreement around the vehicle payments were properly documented and that any conflict of interest was properly addressed.
He said there was no evidence of dishonesty or a lack of integrity.
"The tribunal regards failure to identify and properly manage conflicts of interest as a serious matter," Whale said.
"Of particular note was the member's investment with his client in a company the affairs of which were intertwined with those of another company owned by the member's clients which was in financial difficulty and reliant on bank funding through the joint venture company - the potential for conflict was not addressed by the member until late in the piece," he said.
The fact that a conduct committee cautioned McCarthy in 2014 was an "aggravating factor", he said.
The tribunal censured McCarthy and accepted his undertaking that he would not reapply for a certificate of public practice until next June.
He was ordered to pay $24,000 in costs to the Institute of Chartered Accountants.