It pointed to the prison census data as being unreliable in the sense that it only provides information about prisoners present on the date of the census. (humanrights.gov.au)
In addition, civil liberties group say that facial recognition technology is unreliable because people's facial patterns can change quite quickly. (slaw.ca)
Even under the above assumptions, these figures would still be unreliable as indicators of what participation would have been if the police had not attempted to prevent the referendum. (politics.stackexchange.com)