The head of the Australian Bureau of Statistics has admitted a series of “poor judgments” led to the census bungle, which he predicts will cost taxpayers at least $30 million.

Statistician in charge of the ABS David Kalisch told a Senate Estimates Committee in Canberra on Wednesday that his organisation didn’t undertake enough consultation on privacy concerns before 2016’s online census.

The census website was offline for two days after it opened in August because it was hit with a distributed denial of service (DDOS) – where a site experiences a flood of traffic designed to disrupt it — and technical problems.

“The ABS made a number of poor judgments in our preparation for the 2016 census that led to the poor service experienced by many households,” Mr Kalisch told the committee.

“I apologised to the community on behalf of the ABS, and I repeat that apology sincerely again.”

Mr Kalisch told the committee a second DDOS attempt was made after the site was restored, but he said he was confident the unsuccessful attempt had not compromised any data.

“We did put in place additional protections and have the system to a higher level of robustness, so we were as confident as you could be that the system would be robust,” he said.

The 2016 census had a budget of just under $470 million for five years.

Mr Kalisch said the ABS expected to save $100 million by taking the census largely online, however the bungle had reduced that saving to $70 million.

“We have to date probably incurred additional costs of around $20 million, and these are rough figures, and we anticipate probably spending another $10 million,” he said.

Mr Kalisch assured the committee a “digital first” approach was the best way to move forward.

No fines … yet

ABS officials also confirmed to the committee that nobody had been fined for failing to complete the census.

There had been a campaign by some privacy advocates urging Australians worried about the ABS’s retention of their information to not comply with the census.

The census was a disaster for the ABS and those trying to complete it online. Photo: AAP

People who refuse to fill out the census can be fined $180 per day after a final written warning.

The ABS said it was assessing about 10,000 cases of refusal, but no one had been fined yet.

“At this stage we’ve sent some 1800 refusal letters,” ABS official Chris Libreri said.

“We’ve issued 239 notices of direction, but that’s the only stage we’ve proceeded to at this stage.”

The ABS said 8.4 million forms had been received for a response rate of 96 per cent. The ABS is still accepting paper forms.

‘It’s unacceptable’: Turnbull government

Government frontbencher Zed Seselja admitted errors were clearly made but insisted many lessons had been learnt.

ABS said at the time the census ‘attack’ likely came from abroad. Photo: AAP

“The Prime Minister has made it very, very clear that it’s unacceptable,” he told reporters in Canberra.

“I’m sure the ABS has learnt its lesson.”

Former cabinet minister Eric Abetz said many millions of Australians were inconvenienced by the “debacle”.

“I think responsibility needs to be taken,” he told reporters. “But by whom and in what form, that is to be determined by those that are in charge.”

Mr Kalisch will face more questions at a separate Senate inquiry into the census failure next week.

– with AAP and ABC