Security experts are investigating how Australia's online census was brought to its knees by a hardware failure and cyber attacks.

The federal opposition will seek a Senate inquiry into the bungling of the census when the new parliament sits on August 30.

Prime Minister Malcolm Turnbull said census information entered by 2.33 million people before the system was taken down as a precaution by the Australian Bureau of Statistics was safe despite the denial of service attacks, which came via United States computer networks.

DOS attacks are not designed to steal data but to frustrate systems - they can be likened to parking a car across a driveway, rather than stealing a car.

Mr Turnbull said proper defences were in place, but a computer hardware failure led to the decision to temporarily take down the site.

However he saw no need to re-run the census, which collects vital information every five years.

"I want to assure those Australians who are yet to complete the census that they will be able to do so based on the expert advice we have received, safely and with confidence, that their information that they record, whether it's on paper or online, will be secure," Mr Turnbull told reporters in Sydney on Wednesday.

An initial attack was detected and investigated just after 10am on Tuesday but a larger incident at 7.30pm triggered the ABS to take down the entire system, just as millions of families were sitting down to enter their data.

Australians have until September 23 to complete their forms and won't be fined for late lodgement.

Labor leader Bill Shorten said it was the worst-run census in the nation's history.

"It has taken us 100 years to build confidence in the census. It has taken Malcolm Turnbull one Tuesday night to see this bungle undermine confidence in government institutions," Mr Shorten said.

It was humiliating when the government asked millions of Australians to fill out the census and it couldn't even get that task right.

"Most importantly, we think that the Senate needs to inquire into how this has happened and how can we make sure this doesn't happen again."

The prime minister's cyber security adviser Alastair MacGibbon said most of the online traffic involved in the incident was from the United States, but that was not unusual and did not mean the attacker was US-based.

Mr MacGibbon, who only took on the new role in May, and the Australian Signals Directorate are working with the ABS and Treasury to investigate the matter.

The Australian Privacy Commissioner has also opened an investigation into the ABS and its handling of the attack.

"My first priority is to ensure that no personal information has been compromised as a result of these attacks," commissioner Timothy Pilgrim said in a statement.

Independent senator Nick Xenophon, who had refused to put his name to the census, will be backing a Senate inquiry and questioned how the public still could trust the ABS and its privacy assurances.

He accepted there was no suggestion people's personal data had been hacked, but described it as a "major security failure".

Earlier in the day, ABS chief David Kalisch described it as a malicious "attack", but other government figures declined to describe it as such.

"A denial of service is not a breach, it's not designed to take data. A denial of service is designed to frustrate," Mr MacGibbon said.