Optus’ woes compounded by a weakening Australian economy have forced parent Singtel to write off billions of dollars on the telecommunications group, which has struck a $1.6 billion deal with rival TPG Telecom to bring in cash.

The $S3.1 billion ($3.5 billion) Singtel write-down, revealed early on Monday in Singapore, is mostly related to Optus, which is struggling with falling profits in its enterprise business that provides services to business and government clients and is run by former NSW premier Gladys Berejiklian.