Staff were informed of the decision on Tuesday by chief executive Allen Lew, who said the sackings would be carried out in order to "future-proof" the company.
Virgin Mobile Australia employees would be considered for vacancies in Optus-owned stores and "also be invited to explore opportunities within our group of companies", a spokesperson said at the time.
In the case of the 400 who lose their jobs in this tranche, an Optus spokesperson said, "Our policy is always to speak with employees who may be impacted by these changes first, including discussing redeployment opportunities."
Fairfax Media reported that Optus had 8526 employees as of June 30 last year. This included retail employees and 2000 core staff
Today's cuts are not expected to result in the closure of any retail outlets. Staff from management, administration and redundant staff from other divisions would be hit.
"Optus is making a step-change to the way we work, do business and fund future investments so we can continue to thrive in our increasingly competitive and disruptive market.
"As we make these important strategic decisions, we will ensure we have the right skills and capabilities in place to build our digital organisation, remove duplication and establish a more sustainable cost base.
"We are making a number of changes across the organisation. Certain roles will be impacted either directly or indirectly because of these changes, including approximately 400 roles which will be made redundant."