The chaos surrounding the first online Census which resulted in the website being shut down and thousands of people unable to access it on the night will cost taxpayers about $30m.

The Australian Bureau of Statistics told a Senate estimates hearing on Wednesday night the 40-hour outage meant the expected $100m saving from moving the census online was now more likely to be about $70m.

“We have to date probably incurred additional costs of around $20m ... and we anticipate possibly spending another $10m,” chief statistician David Kalisch said.

The 2016 census has been widely labelled a disaster with privacy fears over increased data retention and the collapse of the online system on census night.

Thousands of people had to complete the census after the designated night, while there were complaints from some who had been asked to fill out paper forms after submitting it online.

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

“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.”

