The Optus decision follows a similar move by Telstra Corp, Australia's largest phone company, which wrote off a A$36 million (US$18.6 million) One.Tel debt earlier this month.
One.Tel, which was in the red for up to A$862 million (US$446.1 million), declared itself insolvent on May 30, and appointed outside administrators.
"Optus will try to recover this money from One.Tel," the country's second largest telco said in a statement. "However, statements by the administrators indicate creditors cannot be certain of recovering any money owed."
Optus said the amount would be written off against profit.
Optus is in the process of being bought over by Singapore Telecoms Ltd in a deal said to be worth A$17 billion (US$8.4 billion).