Reports from Australia say the government has returned almost 2,000 unopened goodwill letters that had been sent to detainees in Nauru last year.
The Guardian Australia says the letters of comfort and support were organised by a Melbourne lawyer, Julian Burnside.
Each letter contained a self-addressed stamped envelope so the detainees could reply to the sender if they wished.
Mr Burnside has received the unopened mail despite the Australian government telling him in June that the letters were being distributed.
Guardian Australia says it also emerged that the Nauruan postal authorities would only accept the self-addressed envelopes if the Australian stamps had been purchased in Nauru.