External consultants called in to help ATO

The tax office has appointed PwC to independently review what went wrong with its online systems last December.

Thousands of Australian Taxation Office clients were left stranded when a failure of storage hardware caused what tax commissioner Chris Jordan described as the "worst unplanned system outage in recent memory".

The subsequent failure of backup systems compounded the initial problem.

The ATO said in a statement on Tuesday PwC had a specific expertise with the ICT storage at the centre of the incident, which was upgraded by Hewlett Packard Enterprise in November 2015.

"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," the agency said.

The PwC review is due to be finalised in March and will be conducted alongside the ATO's own internal review.

All of the ATO's client systems and portals have been restored and further outages are not expected.