The Australian Bureau of Statistics (ABS) and IT giant IBM will today be grilled by a Senate committee over the botched handling of the 2016 census.

A blame game has begun over who was responsible for the 40-hour outage, which saw thousands of Australians being unable to fill out their census forms online on August 9.

The ABS apologised for the incident but also pointed fingers at IBM, claiming it should have done more to prevent the denial of service attack (DDoS) on its website.

IBM blamed subcontractors for the incident, an allegation the companies involved have denied.

Small Business Minister Michael McCormack said the Federal Government would keep a close eye on today's hearings.

"Obviously I'll be paying very close attention to what is said at the Senate inquiry, it is an important inquiry," he said.

"Australians were inconvenienced on the night, there's no doubt about that."

He stressed the census data collected "was safe", following a lengthy investigation by IBM, ABS and the intelligence agency Australian Signals Directorate.

Shadow assistant treasurer Andrew Leigh said the Federal Government had ultimate responsibility for the incident.

"They were the ones who had multiple ministers responsible for the census, they were the ones who cut funding to the ABS in the 2014 budget. And now they're the ones that have to take responsibility for the bungling of the 2016 census," he said.

IT consultant Justin Warren said a lot of questions remained unanswered from the numerous submissions given to the Senate inquiry.

"It hasn't been clear from the answers we've been given so far and we've had a lot of conflicting answers from different people from the Minister, through to IBM, the ABS," he said.

"It's really unclear what actually went on."

Senate Estimates heard the incident will cost taxpayers $30 million.

IBM said it put in place measures to protect against a DDoS by blocking all foreign web traffic to the ABS servers, known as "geo-blocking", which it implemented on the night of the census.

In a submission to a Senate inquiry, it blamed one of its subcontractors for failing to sever one of its international links, allowing the foreign-based DDoS attack to flow through.

It is a claim those subcontractors have denied, who also said alternative DDoS protections were knocked back by IBM.

ABS blamed IBM for "not adequately" addressing the risk of DDoS attacks as part of its contract to the bureau.