Singh will resume work at his Melbourne-based company, Rubicon International, which he founded in 1999. Rubicon also specialises in consulting and project management.
ZDNet Australia understands Singh's resignation took effect in recent weeks.
He has been replaced as CIO by Kerry Crompton, a 25-year veteran of technology and financial services. Crompton was chief information officer and later chief operating officer during his three years at Members Equity Bank. He also spent three years at National Australia Bank in technology roles.
Immediately prior to Medibank, Crompton had worked as an independent management consultant since September 2004.
Crompton started in the role on 3 April.
Crompton will take over the ongoing upgrade of Medibank's technology being undertaken as part of the company's business systems renewal (BSR) program. BSR has seen the company move to upgrade its customer relationship management and data warehousing systems. Medibank separated from the Health Insurance Commission (HIC) in 1998 to become an independent Commonwealth company, and in recent years has moved its mainframe infrastructure from HIC as part of the BSR project.
"We are renewing business technology systems to ensure we are more efficient, competitive, and responsive to members. I'm looking forward to being actively involved in this," Crompton said in a statement.
ZDNet Australia was unable to contact Singh for comment.