Shane Murphy, national president of the Communications, Electrical and Plumbing Union of Australia, said the fresh news of job losses at the second biggest telecommunications outfit in Australia, was a devastating blow to workers and their families.
Optus made the announcement on Tuesday, with its chief executive Allan Lew saying the move had been made to "future-proof" the company.
The announcement came a few days after there was confirmation that Optus would be phasing out its Virgin Mobile subsidiary over the next two years and getting rid of 200 workers in the process.
But on Tuesday, the commitment on this score was less definite, with a spokesperson saying "Our policy is always to speak with employees who may be impacted by these changes first, including discussing redeployment opportunities."
Murphy claimed less than a quarter of the 400 who faced the sack would get an opportunity to be re-employed by Optus.
"While we expect that under 100 of these workers will be offered an opportunity to fill a vacancy elsewhere, there are still over 300 families tonight who will be reeling from this terrible news," he said.
“The union will be doing everything we can to assist the impacted workers during this incredibly difficult time. We will be working will Optus to ensure every single worker gets the entitlements they deserve.
Murphy pointed out that the announcement had been made at a time when the union was getting ready to negotiate a new enterprise agreement with Optus.
“Their timing of these enormous job cuts demonstrates a reprehensible contempt for the Optus workforce," he added.