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.