The SFO said it had determined there were "reasonable grounds to believe" Mr Milne may have committed fraud.
It said it had confirmed its investigation to encourage anyone who may have had financial dealings with Mr Milne to come forward.
The Law Society's Otago standards committee has also asked for people to come forward.
Convener David More yesterday said the committee had taken the "unusual step" of publicising its decision because the complaints were already publicly known, and the committee was not sure if everyone who placed money with Mr Milne had been identified.
Mr Milne ran a sole practice office in Dunedin for many years before taking a role at Craig Paddon Law, where he managed its Christchurch office, known as Harewood Law, from 2008.
He is no longer employed by the firm and has handed in his practising certificate to the Law Society.