Shane Murphy, national president of the Communications, Electrical and Plumbing Union of Australia, said in a statement that the news would hit the workers and their families hard.
Optus confirmed the cuts on Thursday, saying it would cut 200 jobs and close 36 stores as it pulls the plug on its Virgin Mobile brand.
“This is a devastating blow to the 200 workers and their families who have found out today that they no longer have a job,” Murphy said.
“Optus has publicly committed to helping the 200 workers who are now jobless find new work within the company and we’ll certainly be holding them to that.
“We’ll also be working with Optus to ensure every single worker gets the entitlements they deserve.
Murphy said the cuts came only months after Optus slashed 170 jobs in October and on the eve of negotiations for a new Enterprise Agreement at Optus.
“Their timing demonstrates a reprehensible contempt for the Optus workforce," Murphy added.
Virgin Mobile was founded by Sir Richard Branson in 2000 and became a wholly-owned subsidiary of Optus in 2006.
Retaining its own brand, Virgin Mobile offers post-paid mobile and mobile broadband services to about a million Australians.
Optus said on Thursday that it would be contacting Virgin Mobile customers to offer them options as the brand was phased out.