Finlayson was the first senior appointment made by director-general Hamish Pringle when he took over the position from his previous role of director of marketing strategy at the industry body in 2001.
Finlayson said: "I have really enjoyed my time in the UK and in particular my five years with the IPA. "I feel I've learned a lot from one of the most exciting and challenging industries I've worked in, but my heart and my family are in Australia and it's now the right time for me and my partner Jennifer to go home."
The former chartered accountant, previously the finance manager at First Telecom, played a big part in restructuring the finances of the IPA.
Pringle said: "I'm really going to miss Ian and his contribution. He, and the department he has built, have been fundamental to the success of the IPA over the past five years."
During his time at the IPA, Pringle said Finlayson had helped improve the IPA's finances allowing a number of projects to progress, including the refurbishment of its London offices, the opening of a Scottish office and the launch of the IPA TouchPoints survey.
Tony Fenton, the institute's honorary treasurer and MediaCom's finance director, said: "He has increasingly become a valued sounding board for finance directors and other senior figures in the membership. Ian's role in key lobbying activities for the IPA and its membership, such as TV Eye, has been invaluable."
Finlayson will officially leave in October. The IPA will now start the hunt for a successor in the role.
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