The Australian Manufacturing Workers Union (AMWU) says Holden will fail to reach its redundancy target by the end of July.
The car company announced it was shedding 400 jobs at its Elizabeth plant in Adelaide's north, but the AMWU says only 300 workers have agreed to take a redundancy.
Union representatives met Holden management on Friday.
AMWU secretery John Camillo says the future for workers is unclear.
"At this stage we're still saying there won't be any forced redundancies, it should be all voluntary separation," he said.
Holden wants to have the redundancies finalised by the end of next month.
"When the time comes at the end of June or early July we'll sit down and work how we're going to deal with that situation and why they need to reduce further numbers," Mr Camillo said.
"There's still ongoing talks."
The union denied reports that Holden workers are being asked to take a pay cut.