Australia's second-largest telco Optus' Chief Executive Officer Kelly Bayer Rosmarin has resigned, its parent Singapore Telecommunications said on Monday, days after a network-wide outage left nearly half the country without phone or internet for 12 hours.

Optus has appointed Chief Financial Officer Michael Venter as interim CEO. Peter Kaliaropoulos was appointed to a newly created position of chief operating officer, SingTel added.

More than 10 million Australians were hit by the 12-hour network blackout at the nation's second-largest telco for much of Wednesday, triggering fury and frustration among customers and raising wider concerns about the telecommunications infrastructure.

"We recognise the need for Optus to regain customer trust and confidence as the team works through the impact and consequences of the recent outage and continues to improve," SingTel Group CEO Yuen Kuan Moon said in a statement. (Reuters)