The Australian Taxation Office has hired PricewaterhouseCoopers (PwC) to investigate an embarrassing system outage that took down the ATO online services in December 2016.
PwC was selected based on its "specific expertise" with the HPE SAN storage that caused the outage.
The ATO expects the review to be finalised in March.
"The review will help us to fully understand what happened and why, and what needs to be done to ensure we are not exposed to this type of incident in future," an ATO spokesperson said.
There is also an internal review taking place which will focus on the ATO's tax professionals, software developers and other staff.
After being forced to take its services down for three consecutive weekends, the ATO announced that all external-facing client systems and portals have been restored and are operational. The department has no further outages planned for the near future.
The ATO assured that no taxpayer information had been compromised.