The Australian Bureau of Statistics (ABS) has apologised for a mass outage of the census website that prevented thousands of people around the country from completing the survey online for the first time.

The ABS had estimated that two thirds of Australians would fill out the census online this year, rather than on paper.

However on Tuesday night the website crashed, with the ABS shifting into gear to attempt to fix the problem.

The ABC was inundated with reports in the early evening of the census website crashing or failing to load.

"The site just cannot be reached. The phones do not answer and neither does the online messaging," one complainant wrote, while Twitter lit up with the hashtag #CensusFail.

Attempts to access the ABS website by the ABC also failed and calls to the helpline were met with a recorded message stating phone lines would not be answered until August 10.

Late on Tuesday night the ABS admitted defeat, issuing a statement to say the service would not be restored before morning.

The statement said ABS staff would not be available for comment but that a further statement would be issued on Wednesday.

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Small Business Minister Michael McCormack issued a statement to say he had been in constant communication with chief statistician David Kalisch throughout the evening.

"I am informed by the ABS that a thorough process will be undertaken to ensure all households are counted as part of the census," the statement said.

Mr McCormack's statement also said he and Mr Kalisch would address the media about the outage on Wednesday morning.

'Census was v easy' says Turnbull, Twitter disagrees

Having been reminded for weeks to "get online on August 9", many Australians were frustrated to find they could not log on to complete the survey.

Some took aim at the ABS while others questioned whether Australia's broadband network had anything to do with the outage.

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Prime Minister Malcolm Turnbull tweeted that he had completed the census and it was "easy to do".

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In the lead-up to census night, the ABS spent hundreds of thousands of dollars on load testing and said its servers could handle 1 million forms per hour.

Australian Bureau of Statistics boss Chris Libreri earlier said he was confident that the system could handle the increased traffic load expected on census night.

"We have load tested it at 150 per cent of the number of people we think are going to be on it on Tuesday for eight hours straight and it didn't look like flinching," Mr Libreri told News.com.

"We wouldn't do it unless we were able to safely do it, we have evolved it and we are confident."

People officially have until September 23 to complete the census online, and the ABS has said people would not be fined if they did not do it on census night.

"There will be no fines for completing the census after August 9. There's still plenty of time to complete the census. Thanks for your patience," the ABS statement said



The first results from the census will be released in April, two months earlier than the previous census in 2011.