Information Technology experts say a "limited budget" and concerns about keeping data in Australia could have catastrophically combined to bring the Bureau of Statistics census website grinding to a halt on Tuesday night.

The websites for the Census and the Australian Bureau of Statistics went down about 8pm on Tuesday, with the agency saying on Twitter it expected them to remain down until at least Wednesday morning. An estimated 16 million people were expected to log on to complete the census on Tuesday.

Paul Brebner, a Canberra-based software performance expert who is familiar with large-scale public sector websites, said the Bureau of Statistics was prepared for 1 million people to be online simultaneously. However, he performed his own calculations and said they should have been ready for 3 million simultaneous users between the hours of 6pm and midnight.

"You can't assume a uniform load across the 24 hours," he said. "If they were expecting 16 million people over 24 hours, that would be okay, but that's not how loads on websites work at all. There's often one very big spike, and it's hard to see how long it will stay in that level."