An independent expert will investigate the unprecedented meltdown of the Australian Taxation Office's online services.
The ATO's website, tax agent and business portals crashed on Monday after recently-upgraded storage hardware failed, as did back-up arrangements.
Commissioner of Taxation Chris Jordan said on Friday it was the ATO's worst unplanned system outage in recent memory.
"This was an extremely unusual and unfortunate event," he said in a statement.
Mr Jordan said the ATO's failed back-up plan has meant the restoration of data and services has been complex and time consuming but insisted no information had been lost or compromised.
"The issues we have experienced this week do not relate to our overall IT capability or skills," he said.
The ATO has warned of further performance issues over the next couple of days as systems are restored, with staff working over the weekend to catch up.
All refunds scheduled to be issued when the system went down on Monday have now been processed and will be paid by Saturday, with any others in the system to be fast-tracked.
"We have devoted additional resources to ensure the vast majority of people get their refunds on time."
Mr Jordan reiterated that anyone who couldn't make a payment or lodge a form because of the outage won't be penalised.
An independent review into exactly what happened, how and why as well as what measures need to be taken to avoid a repeat has been promised.
It will also look at the ATO's handling of the situation, including communication with the public.
"I will be doing everything I can to learn from what has happened this week and to put in place any necessary changes to minimise the risk of any recurrence," Mr Jordan said.
