The New Zealand-born Ioane played 35 times for the Wallabies and accumulated 86 caps in Super Rugby for the Western Force (2006-07) and Queensland Reds (2008-13) before heading to France and then Japan.
"I have had a great time playing in France and Japan, but the opportunity to be part of a New Zealand Super Rugby team is too good to pass up," Ioane said in a statement after he had signed his two-year contract.
"I love the intensity and tempo of Super Rugby and the New Zealand teams are leading the charge in that respect at the moment, so I can't wait to be a part of that."
Robertson is taking over from Todd Blackadder next season, and needed to bolster his outside backs with wingers Nemani Nadolo and Johnny McNicholl both departing for contracts in Europe.
"Digby is a massive talent, who we believe still has plenty of top class playing years left in him," Robertson said.
"We are excited about what he can do for this team, with his remarkable ability to create opportunities and score tries."
The Crusaders were knocked out of the Super Rugby playoffs last week by South Africa's Lions, with the 42-25 loss ensuring that Blackadder left the club after eight seasons in charge without a title.
(Reporting by Greg Stutchbury; Editing by Sudipto Ganguly)
