Medibank said its Operation Safeguard was completed early on Saturday morning with its systems back online ahead of schedule.
Last week, the company announced the temporary shutdown of its systems so that it could undertake maintenance to enhance security protections.
The shutdown was in response to the illegal access, theft and release of customer information.
On Saturday, the company announced that all customer-facing platforms had been fully tested by IT security experts from Microsoft. They were made operational with enhanced security protections and were back online ahead of schedule at 6.15 am on Saturday 10 December 2022.
"While there has been no further suspicious activity detected inside our systems since 12 October 2022, the maintenance work conducted overnight is part of Operation Safeguard, which includes measures to further strengthen security," said the company.
"This includes the recent addition of two-factor authentication in our contact centres to increase the level of security for our customers when they call for support.
"Since the cybercrime we have bolstered existing monitoring, added further detection and forensics capability across the Medibank system and network and have scaled up analytical support via specialist third parties."