There will be a full independent review into the worst ATO tech crash "in recent memory", the Commissioner of Taxation has pledged.
Chris Jordan's apology and promise of a review came as the ATO's website collapsed again on Friday, with insiders saying "server stability problems" were to blame.
Mr Jordan has promised to get to the bottom of exactly what happened as in-house Tax Office technicians and their private contractors Hewlett Packard continue to work to restore the revenue agency's full IT capabilities with the "unprecedented" disaster in its fifth day.
But the commissioner pledged that no taxpayers would be worse-off as a result of the outage.
"No taxpayers will be disadvantaged as a result of the outage," Mr Jordan said in a statement on Friday.
"This means if you needed to make a payment or lodge a form but couldn't, you won't be penalised."
It is understood that hundreds of IT staff are working "massive hours" at the ATO trying to sort out what is a very complex mess and it is unclear what will happen with the scheduled Christmas shutdown period.
Mr Jordan said in his statement that he believed "normal operations" could resume next week.
"As we return to normal operations next week, we will be fully investigating the events of the last week," he said.
"There will be a comprehensive and independent review to help us answer the questions – exactly what happened, how and why, and what measures need to be taken to avoid such a situation in the future.
"The review will be conducted by an independent expert who will determine the nature of the failure(s) and their root cause(s), the adequacy of back-up and contingency arrangements, and the likelihood of recurrence."