The Australian government says most mail sent to asylum seekers in its detention centre in Nauru last year was returned because they had departed or refused to accept letters from an unknown Australian source.
A barrister in Melbourne, Julian Burnside, had accused the immigration department of intentionally withholding 2,000 letters of support sent by people in Australia to asylum seekers held on Nauru.
The department says the service providers at the Nauru camp attempted to deliver all the letters.
It also says letters were sent to Canberra's detention centre in Papua New Guinea.
The department has not said where the asylum seekers went and why it couldn't forward the mail to those who had departed.
Mr Burnside says the letters were sent by Australians throughout the course of last year, but were returned unopened in bulk to his office in December.
The immigration department says it had been in contact with Mr Burnside and organised the return of all undeliverable letters to him as per his request.
All asylum seekers sent to Nauru and Papua New Guinea have been told by Canberra that they will never be allowed to settle as refugees in Australia because they lodged their application after arriving in Australian waters by boat.