Farquhar joins from MindShare in Sydney, where he held a similar position. Working alongside the planning director, Mark Howley, he will take on the responsibility of running ZenithOptimedia's planning department.
Derek Morris, the agency's chairman and chief strategy officer, will continue in this role. However, Farquhar's appointment will relieve Morris of some of the day-to-day responsibility for running the planning department.
Gerry Boyle, the chief executive of ZenithOptimedia, said: "Stephen's job is to make sure our communications and strategic planning is as strong as it can possibly be. I'm delighted he's come on board."
Farquhar returns to ZenithOptimedia following a stint in Australia. He previously worked at Leo Burnett, Starcom and ZenithOptimedia in the UK. Boyle gave him his first role at Leo Burnett.