The Central Statistics Office has reissued advice to households not to send completed census forms to its Dublin processing unit until the end of the month.
The move follows the receipt of more than 600 completed forms by post since census day last Sunday against advice to retain forms for collection by enumerators.
Mr Aidan Punch, a census co-ordinator and a senior CSO statistician, said some people may have been confused by original instructions.
However, he stressed, "people should keep their completed forms in a safe place at home until an enumerator calls to collect it over the next two to three weeks".
Completed forms should only be posted to the Central Statistics Office's census headquarters if they have not been collected by Monday, May 27th.
Mr Punch said the project was going according to plan, although "there was quite a bit of pressure on our helpline on Monday and Tuesday".
He added it had taken a couple of days to get additional forms out to people who had been missed before census day by enumerators.
Some 4,000 enumerators will be collecting completed forms. Initially, they targeted hotels, guesthouses and transient encampments, "where people were more likely to move on fairly fast", Mr Punch said.
"In the first week or two, we expect to get over 90 per cent back. Mopping up the last 5 per cent will be the difficult job, trying to track down people who are missing or are trying to evade us."
While the Central Statistics Office had guaranteed the information it gathered would not be shared with government departments, "there is a small minority of people who don't seem to believe us", he said.
Completion of the document was compulsory and failure to co-operate can be punished by fines of up to €25,000.
The census helpline can be contacted at 1890 280402.
