The next time you're surveyed by the Bureau of Statistics it'll know more about you than you think you've told it.

The bureau says it'll be able to "enhance" its future surveys by using answers from the census that it believes are from the same person. So if you tell next Tuesday's census your place of birth or religion or family circumstances the answers might be added to the answers you give to future surveys using a "statistical linkage key" that will be created from the name you submit with the census.

For the first time, your name will be held for four years, instead of being destroyed after processing as has happened in the past. However, the linkage keys will be kept indefinitely, meaning the answers to future survey questions can be linked to answers from the census even after the names have been removed.

"There are likely to be some scenarios when it provides a benefit to the people we are surveying," explains the head of the Bureau's census program, Duncan Young. "We won't have to ask for as much information."

Asked whether Australians taking part in door-to-door surveys would be told their answers would be linked to information from their census forms, Mr Young said they might not be told directly.

"That information would be publicly available, but we don't stand at the doorstep and go through every step of our privacy policy with every household," he said. "We make them aware that it's available."

The linkage keys would also be available for use by researchers who wanted to link census records to medical, criminal and administrative records, but the researchers would not be able to see the keys. The bureau would do the linking on their behalf.

"The linkage keys are never released to anyone," Mr Young said. "They will not leave the bureau, even within the bureau we have functional separation which means someone can access names to create a linkage key, someone else can use linkage keys to link data sets together but can't actually access the data sets, and someone else can access the composite data set, so there are three different steps."

Vice-chairman of the Australian Privacy Foundation David Vaile said no matter how securely the keys were stored within the bureau, they had the potential to create a focal point for storing information about individuals.

"One of the concerns we had about the proposal to create an Australia Card identity register in the 1980s was not the idea of the card, but the idea of a single key, a unique number that unlocked the whole thing, that enabled a virtual dossier to be pulled together from all sorts of different data collected for different reasons, with different justifications," he said.

Mr Young said the decision to keep names in order to link census data with other data had not been a particularly notable concern of the people who have been ringing the census hotline.

"That's not why the public are contacting us," he said. "The public are contacting us because they want to get a paper form, they want to know whether they can they complete it before census night, they want to know what they should do if they are out of the country, it's nowhere, it's not even in the top list for us in terms of our public inquiries."

However, former Australian statistician Bill McLennan said the census was "without doubt the most significant invasion of privacy ever perpetrated on Australians by the ABS".

The bureau has contracted IBM to run the website interface meaning that that the 68 per cent of people expected to complete their forms online will have their answers sent to IBM data centres in Australia before being sent to the ABS. Mr Young said the answers would be encrypted from the moment they left each computer and could only be decrypted when they arrived in the ABS. IBM would not hold a decryption key.

More than 200,000 Australians had already submitted their forms and IBM had reported that many more had started filling them out and saved their answers for submitting later.

The online forms were being completed 10 per cent faster than in the 2011 census, perhaps because of faster internet speeds or the improved design. This year's forms work out what questions not to ask and refill answers based on earlier answers to avoid retyping, but they don't make use of information the Bureau already has. "The form is in no way prefilled. When you fill out the form we know absolutely nothing about you," Mr Young said.