- Summary- Companies- Optus hires Deloitte to investigate cyberattack- Optus says 2.1 mln customers may need to replace documents- Singtel lines up lawyersMELBOURNE, Oct 3 (Reuters) - Singapore Telecommunications (STEL.SI) said on Monday it was assessing the potential cost of a massive cybersecurity breach at its Optus arm, Australia's second-largest telco, 12 days ago.At least two major law firms, Slater & Gordon and Maurice Blackburn, have said they are investigating a possible class action against Optus to seek compensation for people affected by the breach."While our overwhelming focus remains on protecting our customers and minimising the harm that might come from the theft of their information, we are determined to find out what went wrong," Optus Chief Executive Officer Kelly Bayer Rosmarin said in a statement released by Singtel.Of the 10 million accounts that were breached, 2.1 million customers had had an identity document number exposed where they may need to take action to replace documents, she said in a video message on the company's web site late on Monday."To our knowledge, there is no other release of the information of the customers that have been exposed," Bayer Rosmarin said."