Riverina households who received a test Census letter have been asked to complete the online or paper survey on Tuesday night.
The Australian Bureau of Statistics (ABS) will run a Census Test night on October 15 to prepare for the real Census in 2021.
ABS Census division general manager Chris Libreri said the test required participants to record who was in their dwelling on that night.
"We hope every household selected completes their Census form fully and accurately, and accounts for everyone in their home, including friends, visitors and newborn babies," he said
"Census data is used by people and organisations from all over Australia to help determine how to tackle issues that impact our lives, and plan for future (health and social) services."
Mr Libreri said the ABS had a commitment to privacy.
"We take your privacy very seriously. We are legally required to keep data secure and ensure it does not disclose any identifiable information about a person or household," he said.
The test area extends from Junee in the north to Holbrook in the South, and from Gundagai in the east to Lockhart in the west.