More than half of Australian households have completed the census, the Australian Bureau of Statistics has revealed, more than a week after the nationwide survey was launched and then shut down for 40 hours following an attempted cyber-attack.

Just over five million have now been submitted online or in paper form, up from the 2.3 million completed on August 9 before the site went down.

The 2011 census achieved a completion rate of 96 per cent of households and it has been reported that forms for at least seven million households - or 10 million people - need to be submitted for the $470 million project to be considered statistically significant.

In a statement released on Wednesday, census chief Duncan Young said the online form had been performing smoothly since being re-established and urged people to submit their forms before the September 23 deadline.

"We thank the many Australians who have already completed the census and we apologise again to everyone for the inconvenience caused on census night," Mr Young said.
 
"If you have not completed the census, we remind you to complete it as soon as possible. Your information is safe and you won't be fined for completing it after census night, but it is important to complete the form either online or in paper as soon as possible."

Former head of the ABS Bill McLennan said on Monday he was "certain" participation rates would not meet targets and warned that even 10 per cent of households failing to complete it would lead to "severe doubts" about its reliability.

The ABS is now mounting an awareness campaign and reminding people not to forget about completing their forms following the botched exercise.