The tech giant responsible for the $30 million census website debacle has admitted the problem could have been averted by simply turning a router off and on again.

IBM’s senior engineer Michael Shallcross told a Senate committee in Canberra this afternoon one of two routers being used on August 9 during census night was not operating correctly, following a series of cyber-attacks.

Mr Shallcross said engineers ran simulations to test what the impact would be if the router was powered down, however nobody actually turned the machine off.

"We tested the router failure by simulating it, which is relatively easy to do and repeatable," Mr Shallcross said.

"That would have discovered earlier that we had that reboot and configuration loading problem."

The error locked Australians out of the census website for more than 40 hours, preventing them from uploading their forms.

Australian Bureau of Statistics (ABS) Statistician David W. Kalisch during a Senate Estimates hearing. (Source: AAP)

The ABS said outage will add an estimated $30 million to the cost of the census.

Chief Statistician David Kalisch from the Australian Bureau of Statistics (ABS) said the committee had been assured by the IBM the system was ready to go on census night.

IBM managing director Kerry Purcell said his company, which was contracted by the Australian Bureau of Statistics to run the eCensus system, took full responsibility for its role in the bungle.

When asked whether any IBM staff have been sacked or discipled over the incident, Mr Purcell replied: "No".

Mr Shallcross said the company had been told by its internet sub-contractors that geo-blocking was properly in place after the third attack on August 9, but it continued to see foreign traffic through Singapore.

The company insisted it anticipated and planned for the risk of DDoS attacks, using protection known as geo-blocking.