"In a short space of time Paul made a huge impact not only on the Millennium Stadium, but also on Welsh life," said Roger Lewis
"He deserves particular recognition for his work in creating the Tsunami Relief Concert and he helped to raise millions of pounds for charity during his time at the stadium.
"Now, though, he is seeking new challenges down under and he leaves with the best wishes and thanks of everyone at the WRU.
Paul Sergeant said of his decision to step down from his role as chief executive of the Millennium Stadium: "As many people are aware, my wife is an Australian citizen, and it's simpy time to go home.
"I've said very openly from day one that its family first and we'll be heading to the Southern Hemisphere as soon as practically possible in the New Year. Wales has been an incredible experience for us and people throughout Wales have been simply awesome.
"I would like to thank the boards of the WRU and the Millennium Stadium, and particularly chairman David Pickering, for their support, advice and encouragement over the past three and a half years.
"In the short space of time I have worked with Roger Lewis I have come to realise what a shrewd appointment this was for the business. There is no one better equipped to drive the Millennium Stadium and the WRU forward.
"My phone line will always be open if need be in the future and I wish him and everyone associated with the business my whole hearted best wishes and support."