The ATO has revealed the cause of an IT outage at tax time last year.

The authorities say a surge in users claiming tax refunds overwhelmed a pair of routers in July last year, knocking ATO Online and the myGov online services portal offline for 16 hours.

Systems including ATO Online, business and tax agent portals and the SBR2 reporting gateway, were forced offline or severely degraded during the outage.

At the time, Services Australia refused to reveal the cause of the outage.

However, the ATO has provided a detailed summary in response to answers to questions on notice from recent budget estimates.

The “likely cause [of the data centre network fault that resulted in the outage] was increased network traffic” the report says.

The ATO said two routers supporting a legacy Teradata environment were the crucial breaking point.

The ATO said it was waiting until after tax time to decommission the system, because it would require a “network wide outage”.

The agency has now begun to decommission the routers, and will scrap the Teradata environment between January and April 2020.