The census website has been restored on Thursday after a day of recriminations with Malcolm Turnbull and senior ministers blaming the Australian Bureau of Statistics and IBM for the failure.

Malcolm Turnbull has said a denial of service attack before the census website outage on Tuesday “should have been repelled” and and has shifted blame to IBM for the systems failure.

But at a press conference in Canberra, Turnbull conceded the denial of service attack did not cause the systems failure, but rather apparently “anomalous traffic” which led the Australian Bureau of Statistics to take the census down.

In an interview with 2GB radio Turnbull revealed the census website should be back online on Thursday. The site was restored shortly before 3pm, 43 hours after it was taken down at 7:45pm on Tuesday evening. However some users are reporting delays in trying to access the site as the site comes back online.

The Census website is now available. Thanks for your patience. We apologise for the inconvenience. https://t.co/j03F1bkPGl — Census Australia (@ABSCensus) August 11, 2016

David Kalisch, head of the ABS, apologised for the inconvenience at a press conference later on Thursday but said, with the census form now back online, “we are on track to conduct a high-quality online census”.

He did not take questions.

The immigration minister, Peter Dutton, said IBM had “questions to answer” about why measures to stop the attack weren’t in place.

The comments contrast with the first explanation provided by the small business minister, Michael McCormack, who said on Wednesday the ABS’s “overcautious” approach, taking the system offline after the technical failure, was to blame.

Cybersecurity chief Alastair MacGibbon warned on Thursday the census website was likely to face even more vigorous denial of service attacks when it was restored.

Turnbull said: “Measures that ought to have been in place to prevent these denial of service attacks interfering with access to the website were not put in place – that is a fact, that was a failure.”

He explained that was compounded by a failure in hardware and “inadequate redundancy” a further indication the capacity of the system itself rather than the scale of the attack was to blame. Turnbull said the failures had now been rectified and the site would probably be restored on Thursday.

Turnbull said denial of service attacks were “completely predictable” and “should have been repelled readily”. “They weren’t because of the failure of systems put in for the [Australian Bureau of Statistics] by IBM.”

He said there were “clearly very big issues for IBM – the provider of the systems – and the ABS itself”.

At a press conference in Canberra, Turnbull blamed hardware failure and “anomalous traffic on the night – it appeared to be anomalous actually it was quite innocent it turned out – but that caused the ABS to take the site down”.

“The site was not crashed by denial of service but it was a confluence of events that caused the ABS to make that decision.

“But there is no doubt there was a failure on the part of ABS and the systems provider.”

In the radio interview, Turnbull said he was “very angry and bitterly disappointed” and said the root cause of the problem was “measures that ought to have been in place” that were not.

Asked if anyone would be sacked over the census failure, Turnbull predicted there would be “serious consequences” after a review by MacGibbon.

When radio host Alan Jones singled out former assistant minister to the treasurer Alex Hawke for criticism, Turnbull said the issue of “which heads should roll, where and when” would be decided after the review.

The shadow finance minister, Jim Chalmers, told ABC’s AM radio program Turnbull’s comments were “unseemly finger pointing after the event”.

“I thought it was pretty pathetic to hear the prime minister blaming the ABS for what is essentially a failure of leadership on his part,” he said.

Speaking on 2GB with Ray Hadley on Thursday, Dutton said the ABS and McCormack had “done their best” and “lessons were learned”.

“The contractors, IBM, will have questions to answer about why the appropriate measures weren’t put in place.”

At a press conference in Melbourne, Bill Shorten, said: “The census happens every five years. It costs hundreds of millions of dollars. There are ministers responsible for the census.”

“Now we wake up Thursday morning and Malcolm Turnbull and his whole Liberal-National ministry front-bench, none of them are apparently responsible for anything. How can it be?”

In another sign of anger at IBM, former Queensland premier Campbell Newman tweeted a link to an article about his government blacklisting IBM from future state contracts in 2013 after a scathing report into the IT giant’s part in the state’s health payroll bungle.

The comments are likely to draw attention to the capacity of the system provided by IBM to the ABS and the specifications required by the contract for it.

On Wednesday, cyber security expert Suelette Dreyfus told the ABC’s The Drum program IT contractors were not necessarily to blame for systems failures.

“Sometimes governments understate the size of what it is they say that they want. Because they haven’t scoped it properly or because they don’t want to pay for it properly,” she said.

On ABC News Breakfast, MacGibbon said the ABS had taken some time to prepare the site for further attacks, including providing “a lot more redundancy and some greater depth to the security”.

“We shouldn’t have had those types of hardware failures.

“We should have been able to make sure that we had redundancies, so anyone that’s ever run an IT system will tell you from time to time, these computers and boxes fall over.”

Asked if there would be further denial of service attacks when the site was restored on Thursday, MacGibbon said: “I think it’s only logical that that’s going to happen. Probably with more vigour than we saw a few nights ago.”

MacGibbon said every effort had been made to ensure the site was robust, but there were no absolute guarantees it would not suffer a further failure.

Jones also grilled Turnbull over whether it was “duplicitous” of the treasurer, Scott Morrison, to approve sale of Carlton Hill and Ivanhoe agricultural land to foreign owners before the election without telling voters.

Turnbull said he would “resist that innuendo or allegation” and did not have details of the sales to hand.

He said the foreign investment review board decisions were “rigorous and objective” and directed further questions to Morrison.

Guardian Australia is seeking a response from IBM.