ABS won't reveal exact census numbers

The government agency collecting the nation's statistics doesn't know or won't say exactly how many census forms have been completed since a website it took down went back online a week ago.

The Australian Bureau of Statistics says more than half of Australian households have now filled out the survey, but won't reveal how many forms have been completed online.

About 2.3 million forms were submitted on census night before the census website was shut down after a series of cyber attacks.

There are concerns some Australians are prepared to risk $180 per day fines and boycott or mislead the census amid privacy concerns.

The ABS wants to keep names and addresses for four years to link them to other government records to collect more accurate data.

Several senators have vowed not to detail their names and addresses on their census form.

An ABS spokesman told AAP on Thursday the number of forms collected so far, both online and in paper form, was similar to the census in 2011 at the same point.

Reminder letters will be sent to households from this week.

The ABS has 38,000 field officers visiting households and maintains the census is progressing well.