​PwC appointed to review ATO's 'worst ever' unplanned outage

The Australian Taxation Office (ATO) announced on Tuesday it has appointed PwC to conduct an independent review into the "world first" hardware issue the ATO experienced in December.

The tax office expects the consultancy firm's review will provide insight into what actually happened and why, and what needs to be done to ensure the same incident does not occur in the future.

The ATO said it chose PwC to conduct the independent review due to the firm's "specific expertise" with the storage hardware at the centre of the incident.

The storage hardware in question was upgraded in November 2015 by Hewlett-Packard Enterprise (HPE) and was seen by the ATO to be "state-of- the-art" at the time, with the ATO noting last year it was "basically the same" hardware used by other large clients of HPE.

In addition to the PwC review, which is to be finalised in March, the ATO will be conducting its own internal investigation which it said will focus on tax professionals and software developers.

On Thursday, the ATO said it was expecting disruptions to its online systems as it looked to continue to fix the problems that emerged last year, after a business reporting system known as SBR1 failed a week prior.

A spokeswoman for the ATO told AAP at the time that the event was merely a "minor disruption", but the tax office took its systems offline that weekend to conduct "critical system maintenance" as it sought to restore systems to full functionality.

The agency also undertook a significant amount of work over the Christmas break to ensure its website and portals were functioning properly.

"All of our external facing client systems and portals have been restored and are operational," the ATO said in a statement on Tuesday. "We do not anticipate any further outages in the short term."

The ATO's website, tax agent, and business portals initially crashed on December 12 as a result of a hardware issue. The outage continued through to December 13, when the ATO called in HPE to help it determine the underlying cause of the problem that the ATO said was encountered for the first time anywhere in the world.

Three days later, Commissioner of Taxation Chris Jordan announced the independent review into the "unprecedented failure" and called it the ATO's worst unplanned system outage in recent memory.

"This was an extremely unusual and unfortunate event," he said in a statement. "The issues we have experienced this week do not relate to our overall IT capability or skills."

At the same time, the ATO said almost everything was back up and running, but did admit that "some" data corruption was experienced as a result of the hardware-related incident, and noted it was in the process of having the data fully restored from a back-up.

On December 20 -- more than a week after the storage hardware failure caused the crash of its online services -- the ATO said it was still experiencing reduced functionality across some of its systems.

"What compounded the problem beyond the initial failure was the subsequent failure of our back-up arrangements to work as planned," Jordan explained previously. "The failure of our back-up arrangements meant that restoration and resumption of data and services has been very complex and time consuming."